feat(channel-discovery): add purpose column + PATCH /api/channels/:id

Adds a free-form 'purpose' text field on Channel so agents (or anyone
creating a channel via API) can describe what the channel is for —
'debate broadcasts', 'security alerts', etc. — and other agents can
later find the right channel by intent rather than channel id.

Wire:
  - Channel.purpose (text, nullable; TypeORM synchronize auto-adds)
  - POST /api/channels accepts optional 'purpose' in body
  - GET /api/channels returns purpose on every row (already returns the
    full entity via {...c})
  - PATCH /api/channels/:id { purpose } — member-or-public auth (mirrors
    the close() rule). Today only 'purpose' is patchable; other fields
    would get their own typed branch.

Frontend create form continues to omit the field — purpose stays optional.
This pairs with Fabric.OpenclawPlugin's fabric-channel-set-purpose tool +
fabric-channel-list returning purpose, so agent workflows can say 'find
an announce channel about X' instead of pinning a UUID.

src/channels/channels.controller.ts

@@ -1,4 +1,4 @@
-import { Body, Controller, Get, Param, Post, Query, Req, UnauthorizedException } from '@nestjs/common';
+import { BadRequestException, Body, Controller, Get, Param, Patch, Post, Query, Req, UnauthorizedException } from '@nestjs/common';
 import { ChannelsService } from './channels.service.js';
 
 // ApiKeyGuard attaches the introspected Center user id onto the request.
@@ -32,11 +32,33 @@ export class ChannelsController {
         bypassUserIds: Array.isArray(body.bypassUserIds)
           ? (body.bypassUserIds as string[])
           : [],
+        purpose: body.purpose as string | undefined,
       },
       userId,
     );
   }
 
+  // Patch a channel's free-form purpose. Body: { purpose: string }. Pass
+  // empty string to clear. Auth: channel member (or anyone for public
+  // channels, mirroring close()). Frontend doesn't call this today —
+  // intended for agent-side use (fabric-channel-set-purpose tool).
+  @Patch(':id')
+  patch(
+    @Req() req: AuthedRequest,
+    @Param('id') channelId: string,
+    @Body() body: Record<string, unknown>,
+  ) {
+    const userId = req.userId ?? '';
+    if (!userId) throw new UnauthorizedException('missing user');
+    // Only `purpose` is patchable today. Future patchable fields would
+    // get their own typed branch; we explicitly NOT allow {} no-op patches
+    // because that signals a caller bug.
+    if (typeof body.purpose !== 'string') {
+      throw new BadRequestException('purpose (string) is required');
+    }
+    return this.channelsService.updatePurpose(channelId, userId, body.purpose);
+  }
+
   // Move an order member into the bypass list (discuss/work only).
   @Post(':id/bypass')
   bypass(

src/channels/channels.service.ts

@@ -24,6 +24,10 @@ type CreateChannelInput = {
   // discuss/work only: members excluded from rotation (no wakeup unless
   // @-mentioned). order and bypass partition the members disjointly.
   bypassUserIds?: string[];
+  // Free-form description of what this channel is for. Optional; agents
+  // typically fill it when creating, members can later edit via
+  // PATCH /api/channels/:id.
+  purpose?: string;
 };
 
 @Injectable()
@@ -166,6 +170,9 @@ export class ChannelsService {
     // allowed (create() always makes a fresh one, no dedup).
     const isPublic = xType === 'dm' ? false : Boolean(input.isPublic);
 
+    const purposeRaw = String(input.purpose ?? '').trim();
+    const purpose = purposeRaw === '' ? null : purposeRaw;
+
     const channel = await this.channelRepo.save(
       this.channelRepo.create({
         guildId,
@@ -174,6 +181,7 @@ export class ChannelsService {
         kind: input.kind === 'announcement' ? 'announcement' : 'text',
         isPrivate: !isPublic,
         isPublic,
+        purpose,
         lastSeq: 0,
       }),
     );
@@ -219,6 +227,27 @@ export class ChannelsService {
     return channel;
   }
 
+  // Update a channel's free-form purpose. Any channel member may do this
+  // (or any guild user if the channel is public, mirroring closeChannel's
+  // member-or-public rule). Pass an empty string to clear.
+  async updatePurpose(channelId: string, actorUserId: string, purpose: string) {
+    const channel = await this.channelRepo.findOne({ where: { id: channelId } });
+    if (!channel) throw new NotFoundException('channel not found');
+    const member = await this.memberRepo.findOne({ where: { channelId, userId: actorUserId } });
+    if (!member && !channel.isPublic) {
+      throw new ForbiddenException('not a channel member');
+    }
+    const trimmed = String(purpose ?? '').trim();
+    channel.purpose = trimmed === '' ? null : trimmed;
+    const saved = await this.channelRepo.save(channel);
+    return {
+      id: saved.id,
+      name: saved.name,
+      xType: saved.xType,
+      purpose: saved.purpose,
+    };
+  }
+
   // Move an order member into the bypass list (discuss/work only).
   // Any channel member may do this.
   async moveToBypass(channelId: string, actorUserId: string, targetUserId: string) {

src/entities/channel.entity.ts

@@ -23,6 +23,16 @@ export class Channel {
   @Column({ type: 'varchar', length: 16, default: 'text' })
   kind!: 'text' | 'announcement';
 
+  // Free-form description of what this channel is for — what topics get
+  // posted, who participates, why it exists. Surfaced via GET /api/channels
+  // so agents can pick a channel by intent ("which announce channel is for
+  // debate broadcasts?") without channel id hard-coded into workflows.
+  // Any channel member can set it via PATCH /api/channels/:id (writes
+  // require membership the same way moveToBypass / close do). The frontend
+  // create form does NOT post this today — purpose stays optional.
+  @Column({ type: 'text', nullable: true })
+  purpose!: string | null;
+
   @Column({ type: 'boolean', default: false })
   isPrivate!: boolean;