feat(guild): announce channel type + agent-presence + busy-discard

Phase 1 of DIALECTIC-V2 — adds Fabric infrastructure for
system-broadcast channels with HF-status-aware delivery filtering.

New channel x_type 'announce':
- channels.entity.ts + channels.service.ts + realtime.gateway.ts
  enum + union extended.
- computeDelivery() adds an 'announce' case: recipient with
  presence='busy' → 'skip' (discarded silently); other presences →
  'observer' (delivered, no wake). System-broadcast semantics —
  agents proactively check their announce inbox when they're ready,
  not interrupted out of band.
- messaging.controller POST guard: announce-type channels reject
  posts that don't present x-fabric-system-key header matching
  FABRIC_BACKEND_GUILD_SYSTEM_API_KEY env. Empty env = no system
  caller is valid (closed-by-default).

New entity + module agent_presences:
- agent-presence.entity.ts: per-user (userId PK) status enum
  (idle/on_call/busy/exhausted/offline/unknown), source tag, updatedAt
- agent-presence.service.ts: getStatus/getStatusMap (bulk for
  delivery-time fanout) + setStatus (upsert)
- agent-presence.controller.ts: GET + PUT /agents/:userId/presence
- agent-presence.module.ts: TypeORM forFeature + wired into AppModule
- buildTypeOrmConfig() entities list extended

RealtimeGateway wiring:
- New optional  field on the gateway (typed loosely to avoid
  circular import). RealtimeModule.onModuleInit() assigns from the
  injected AgentPresenceService — degrades gracefully (no busy-discard,
  treat all as 'unknown') if presence wiring is ever removed.
- emitMessageCreated pre-loads presence per fanout only when xType is
  'announce' (other xTypes bypass the lookup entirely).

Note: actual presence data writes come from Fabric.OpenclawPlugin's
presence-sync loop (separate commit on that submodule); without it,
all rows are 'unknown' and announce delivery falls through to the
default observer behavior (no busy filtering). System-only POST gate
is independent and works immediately.

See /home/hzhang/arch/DIALECTIC-V2-DESIGN.md sections 7 + 10 Phase 1.

src/agents/agent-presence.service.ts

@@ -0,0 +1,53 @@
+import { Injectable } from '@nestjs/common';
+import { InjectRepository } from '@nestjs/typeorm';
+import { Repository } from 'typeorm';
+import { AgentPresence } from '../entities/agent-presence.entity.js';
+
+export type PresenceStatus = 'idle' | 'on_call' | 'busy' | 'exhausted' | 'offline' | 'unknown';
+
+@Injectable()
+export class AgentPresenceService {
+  constructor(
+    @InjectRepository(AgentPresence)
+    private readonly repo: Repository<AgentPresence>,
+  ) {}
+
+  /**
+   * Get a user's current presence. Returns 'unknown' if no row.
+   * Used by `RealtimeGateway` per-recipient when xType === 'announce'.
+   */
+  async getStatus(userId: string): Promise<PresenceStatus> {
+    if (!userId) return 'unknown';
+    const row = await this.repo.findOne({ where: { userId } });
+    return row?.status ?? 'unknown';
+  }
+
+  /** Bulk variant for delivery-time lookups across many recipients in one trip. */
+  async getStatusMap(userIds: string[]): Promise<Map<string, PresenceStatus>> {
+    const out = new Map<string, PresenceStatus>();
+    for (const id of userIds) out.set(id, 'unknown');
+    if (userIds.length === 0) return out;
+    const rows = await this.repo
+      .createQueryBuilder('p')
+      .where('p.userId IN (:...ids)', { ids: userIds })
+      .getMany();
+    for (const r of rows) out.set(r.userId, r.status);
+    return out;
+  }
+
+  /**
+   * Upsert a user's presence. Source is a free-text tag for debugging
+   * (e.g. "hf-plugin", "manual", "test"). PUT /agents/:id/presence
+   * calls this; the plugin pushes only on diff so writes are sparse.
+   */
+  async setStatus(userId: string, status: PresenceStatus, source: string): Promise<AgentPresence> {
+    const existing = await this.repo.findOne({ where: { userId } });
+    if (existing) {
+      existing.status = status;
+      existing.source = source;
+      return this.repo.save(existing);
+    }
+    const row = this.repo.create({ userId, status, source });
+    return this.repo.save(row);
+  }
+}