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/entities/channel.entity.ts

@@ -16,9 +16,9 @@ export class Channel {
   @Column({
     name: 'x_type',
     type: 'enum',
-    enum: ['general', 'work', 'report', 'discuss', 'triage', 'custom', 'dm'],
+    enum: ['general', 'work', 'report', 'discuss', 'triage', 'custom', 'dm', 'announce'],
   })
-  xType!: 'general' | 'work' | 'report' | 'discuss' | 'triage' | 'custom' | 'dm';
+  xType!: 'general' | 'work' | 'report' | 'discuss' | 'triage' | 'custom' | 'dm' | 'announce';
 
   @Column({ type: 'varchar', length: 16, default: 'text' })
   kind!: 'text' | 'announcement';