feat(guild): restore system-key bypass + isSystem msg path

Resurrects the x-fabric-system-key bypass + isSystem branch on POST
/channels/:id/messages, dropped in ca20df7 when dialectic stopped
broadcasting topic lifecycle events to Fabric. Re-enabling now because
Fabric.OpenclawPlugin's close-sub-discussion needs to write a callback
into a parent channel as a system-authored message (not as the closing
host), with an optional precision wakeup so the recruitment workflow can
resume immediately after an interview sub-discussion closes.

Three coupled bits:

1. ApiKeyGuard pre-Bearer bypass: when x-fabric-system-key matches
   FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY, set req.isSystem=true and
   skip the Bearer check. Intentionally reuses the existing commands
   sync env — same shared secret, same consumer (the OpenclawPlugin
   reads channels.fabric.commandsSyncKey for both paths). One less env
   to rotate, one less secret to manage.

2. messaging.controller POST /channels/:id/messages adds an isSystem
   branch (runs before the participant gate):

   - Looks up the channel directly (not via assertParticipant).
   - Persists with sentinel author 00000000-0000-0000-0000-000000000000,
     same UUID the old impl used.
   - Translates <@user.name:NAME> mentions like the regular path.
   - When wakeupUserId is set, delivers via emitMessageTargeted so that
     exactly that one recipient receives wakeup=true; everyone else gets
     wakeup=false. When omitted, delivers via emitMessageCreated with an
     empty wakeUserIds set so nobody is woken — silent system log.

   Two intentional differences from the 985b06a original:
   - No xType=announce restriction. The original was limited to announce
     because that was Dialectic's only use case; we now need this on dm /
     general / discuss / etc. for the sub-discussion callback. Closed
     channels are still rejected (409) on both paths.
   - The wakeupUserId field is new — old impl only ever sent silent
     announces.

3. DTO carries wakeupUserId? optional string. Ignored on the regular
   user-bearer path; load-bearing on the system path.

Shared helper: extracted commands.controller's private safeEqual into
src/common/safe-equal.ts so api-key.guard.ts can use the same constant-
time check. Vitest spec covers equal / inequal / length-mismatch / empty
cases. Existing unit tests still pass.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/commands/commands.controller.ts

@@ -8,19 +8,12 @@ import {
   Req,
   UnauthorizedException,
 } from '@nestjs/common';
-import { timingSafeEqual } from 'node:crypto';
 import { CommandsService } from './commands.service.js';
 import { SyncCommandsDto } from './dto.sync-commands.dto.js';
+import { safeEqual } from '../common/safe-equal.js';
 
 type AuthedRequest = { userId?: string };
 
-function safeEqual(a: string, b: string): boolean {
-  const ab = Buffer.from(a);
-  const bb = Buffer.from(b);
-  if (ab.length !== bb.length) return false;
-  return timingSafeEqual(ab, bb);
-}
-
 @Controller('commands')
 export class CommandsController {
   constructor(private readonly commands: CommandsService) {}

src/common/api-key.guard.ts

@@ -5,6 +5,7 @@ import {
   UnauthorizedException,
 } from '@nestjs/common';
 import { introspectGuildToken } from './center-auth.js';
+import { safeEqual } from './safe-equal.js';
 
 @Injectable()
 export class ApiKeyGuard implements CanActivate {
@@ -21,6 +22,25 @@ export class ApiKeyGuard implements CanActivate {
       return true;
     }
 
+    // System-key bypass: when a caller presents x-fabric-system-key
+    // matching FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY (intentionally the
+    // same shared secret as x-commands-sync-key — both legitimate
+    // consumers are Fabric.OpenclawPlugin), skip the Bearer requirement
+    // and mark this as a system caller. Downstream handlers (e.g.
+    // messaging.controller POST /channels/:id/messages) gate on
+    // req.isSystem to take the system-author code path.
+    //
+    // Empty env → bypass disabled (no system caller ever valid; closed
+    // by default). Header carries the secret as-is; we constant-time
+    // compare against the env value.
+    const sysExpected = (process.env.FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY ?? '').trim();
+    const sysHeader = req.headers['x-fabric-system-key'];
+    const sysProvided = Array.isArray(sysHeader) ? sysHeader[0] : sysHeader;
+    if (sysExpected && sysProvided && safeEqual(sysProvided, sysExpected)) {
+      (req as { isSystem?: boolean }).isSystem = true;
+      return true;
+    }
+
     const auth = req.headers['authorization'];
     const authValue = Array.isArray(auth) ? auth[0] : auth;
     let token = authValue?.startsWith('Bearer ') ? authValue.slice(7) : '';

src/common/safe-equal.spec.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect } from 'vitest';
+import { safeEqual } from './safe-equal.js';
+
+describe('safeEqual', () => {
+  it('returns true for identical non-empty strings', () => {
+    expect(safeEqual('abc123', 'abc123')).toBe(true);
+  });
+
+  it('returns false for different strings of same length', () => {
+    expect(safeEqual('abc123', 'abc124')).toBe(false);
+  });
+
+  it('returns false for differing lengths', () => {
+    expect(safeEqual('abc', 'abcd')).toBe(false);
+  });
+
+  it('handles empty strings', () => {
+    // both empty is technically equal — but downstream callers should
+    // explicitly check for empty expected before invoking. We just
+    // document the constant-time-comparison primitive's behavior.
+    expect(safeEqual('', '')).toBe(true);
+    expect(safeEqual('a', '')).toBe(false);
+    expect(safeEqual('', 'a')).toBe(false);
+  });
+});

src/common/safe-equal.ts

@@ -0,0 +1,12 @@
+import { timingSafeEqual } from 'node:crypto';
+
+// Constant-time string comparison. Returns false for length mismatch (the
+// length difference itself is observable, but the per-byte loop isn't).
+// Used for shared-secret header checks (commands-sync-key, system-key,
+// etc.) to keep timing-oracle attacks off the table.
+export function safeEqual(a: string, b: string): boolean {
+  const ab = Buffer.from(a);
+  const bb = Buffer.from(b);
+  if (ab.length !== bb.length) return false;
+  return timingSafeEqual(ab, bb);
+}

src/messaging/dto.create-message.dto.ts

@@ -56,4 +56,14 @@ export class CreateMessageDto {
   @IsString()
   @MaxLength(64)
   authorUserId?: string;
+
+  // System-author path only (x-fabric-system-key gated). When set, the
+  // message is delivered via emitMessageTargeted so this single recipient
+  // gets wakeup=true; everyone else in the channel sees wakeup=false. For
+  // regular (user-bearer) posts this field is ignored. Used by
+  // close-sub-discussion to precisely wake the host on callback.
+  @IsOptional()
+  @IsString()
+  @MaxLength(64)
+  wakeupUserId?: string;
 }

src/messaging/messaging.controller.ts

@@ -156,13 +156,70 @@ export class MessagingController {
   async create(
     @Param('id') channelId: string,
     @Body() body: CreateMessageDto,
-    @Req() req: { userId?: string },
+    @Req() req: { userId?: string; isSystem?: boolean },
     @Headers('idempotency-key') idempotencyKey?: string,
   ) {
     const scope = `POST:/channels/${channelId}/messages`;
     const existed = await this.getIdempotentResponse(scope, idempotencyKey);
     if (existed) return existed;
 
+    // System caller (ApiKeyGuard set isSystem from x-fabric-system-key):
+    // skip the per-user participant check; resolve channel directly.
+    // System posts are allowed into any non-closed channel — used by
+    // Fabric.OpenclawPlugin to write `close-sub-discussion` callbacks
+    // back to a parent channel that the host agent may not be currently
+    // "in" from the backend's perspective, and to deliver guide-injected
+    // system intros into sub-discussion channels without needing to log
+    // in as a real user. Author is a sentinel UUID that no real user
+    // ever has; `wakeupUserId` (optional) lets the caller precisely wake
+    // one recipient (e.g. the host of a closing sub-discussion).
+    if (req.isSystem) {
+      const sysChannel = await this.channelRepo.findOne({ where: { id: channelId } });
+      if (!sysChannel) throw new NotFoundException('channel not found');
+      if (sysChannel.closed) {
+        throw new ConflictException({ error: 'channel_closed', message: 'channel is closed' });
+      }
+      const SYSTEM_USER_ID = '00000000-0000-0000-0000-000000000000';
+      let sysContent = body.content ?? '';
+      const sysNames = extractNameMentions(sysContent);
+      if (sysNames.length) {
+        const nameMap = await resolveUserNames(sysNames);
+        sysContent = replaceNameMentions(sysContent, nameMap);
+      }
+      const sysMessage = await this.persistMessage(channelId, {
+        authorUserId: SYSTEM_USER_ID,
+        content: sysContent,
+        clientMessageId: body.clientMessageId,
+        replyToMessageId: body.replyToMessageId,
+        mentions: body.mentions,
+        attachments: body.attachments,
+      });
+      const sysView = this.toView(sysMessage) as Record<string, unknown>;
+      await this.saveIdempotentResponse(scope, idempotencyKey, sysView);
+      await this.events.emit({
+        eventType: 'message.created',
+        channelId,
+        actorId: SYSTEM_USER_ID,
+        data: sysView,
+      });
+      // wakeupUserId set -> emitMessageTargeted wakes exactly that user
+      // (everyone else gets the same message with wakeup=false).
+      // wakeupUserId omitted/null -> emitMessageCreated routes via the
+      // channel's xType-specific 3-state delivery with empty wakeSet, so
+      // nobody is woken (the message lands in history only).
+      const wakeupUserId = typeof body.wakeupUserId === 'string' ? body.wakeupUserId.trim() : '';
+      if (wakeupUserId) {
+        await this.realtime.emitMessageTargeted(channelId, sysView, wakeupUserId);
+      } else {
+        await this.realtime.emitMessageCreated(channelId, sysView, {
+          xType: sysChannel.xType ?? 'general',
+          authorUserId: SYSTEM_USER_ID,
+          wakeUserIds: new Set<string>(),
+        });
+      }
+      return sysView;
+    }
+
     // Guild C-1: caller must be a participant of the channel, and the
     // author is always the authenticated user — body.authorUserId is
     // ignored so a caller can never post as someone else.