feat(plugin): add presence-sync module (Phase 1 partial wire)

Drops the PresenceSync class file under src/. Reads each agents HF
status from globalThis.__hfAgentStatus (exposed by
HarborForge.OpenclawPlugin) every 30s and PUTs deltas to
Fabric.Backend.Guild PUT /agents/:userId/presence so the backend can
do busy-discard on announce channel deliveries.

Implementation:
- Diffs against in-memory lastStatus map per agentId; PUT only on
  change. No-op when __hfAgentStatus is undefined (HF plugin not
  loaded) — degrades gracefully, backend defaults presence to
  unknown which means no busy filtering.
- Per-account context: {agentId, fabricUserId, guildBaseUrl,
  fabricApiKey}. Uses x-api-key header so it goes through the
  existing ApiKeyGuard path on the backend.

NOT YET WIRED into index.ts gateway_start lifecycle. To finish
wiring, the registerFull block needs to:
  1. After FabricInbound.start() resolves, harvest each agents
     fabric user id (introspected by Center during session login —
     available on FabricSession.user.id).
  2. Build PresenceSyncAccount[] from those + the existing accounts
     list (which already has agentId + fabricApiKey + guildBaseUrl).
  3. presence = new PresenceSync(api.logger); presence.setAccounts(...);
     presence.start();
  4. presence.stop() on gateway_stop.

Reason for splitting: wiring needs the FabricInbound public API to
expose per-account session metadata, which is a small but separate
refactor. Module ships standalone now so the dependency direction is
clear and the wire-up patch is small.

See /home/hzhang/arch/DIALECTIC-V2-DESIGN.md section 7 (resolved
push-model design).

src/presence-sync.ts

@@ -0,0 +1,92 @@
+/**
+ * presence-sync — read each connected agent's HF status (via the
+ * cross-plugin `globalThis.__hfAgentStatus.get(agentId)` exposed by
+ * HarborForge.OpenclawPlugin) and push diffs to Fabric.Backend.Guild
+ * `PUT /agents/:userId/presence` so the backend can apply busy-discard
+ * on `announce`-type channel deliveries.
+ *
+ * Push model: we only PUT when an agent's status actually changes
+ * (since the last push). The HF-side accessor has its own TTL cache
+ * to absorb the every-30s polling.
+ *
+ * If HF plugin isn't loaded (`__hfAgentStatus` undefined), the loop
+ * is a no-op — Fabric backend defaults presence to 'unknown' which is
+ * treated as not-busy. Announce-channel delivery still works; busy
+ * filtering simply doesn't kick in.
+ */
+
+type HfStatus = 'idle' | 'on_call' | 'busy' | 'exhausted' | 'offline';
+type Bridge = { get(agentId: string): Promise<HfStatus | undefined> };
+type Logger = { info: (m: string) => void; warn: (m: string) => void };
+
+export interface PresenceSyncAccount {
+  agentId: string;
+  fabricUserId: string; // the agent's Fabric Center user id (UUID)
+  guildBaseUrl: string; // e.g. https://fabric.hangman-lab.top/guild/<id>
+  fabricApiKey: string; // existing per-account key
+}
+
+export class PresenceSync {
+  private timer: ReturnType<typeof setInterval> | null = null;
+  private readonly lastStatus = new Map<string, HfStatus>(); // by agentId
+  private readonly accounts = new Map<string, PresenceSyncAccount>();
+
+  constructor(private readonly logger: Logger) {}
+
+  setAccounts(accounts: PresenceSyncAccount[]): void {
+    this.accounts.clear();
+    for (const a of accounts) this.accounts.set(a.agentId, a);
+  }
+
+  start(intervalMs = 30_000): void {
+    if (this.timer) return;
+    this.timer = setInterval(() => {
+      this.tick().catch((err) => this.logger.warn(`fabric: presence-sync error: ${String(err)}`));
+    }, intervalMs);
+    // run once immediately so initial state lands fast
+    void this.tick();
+  }
+
+  stop(): void {
+    if (this.timer) {
+      clearInterval(this.timer);
+      this.timer = null;
+    }
+  }
+
+  private async tick(): Promise<void> {
+    const bridge = (globalThis as Record<string, unknown>)['__hfAgentStatus'] as Bridge | undefined;
+    if (!bridge || typeof bridge.get !== 'function') return; // HF plugin not loaded — skip
+
+    for (const [agentId, acct] of this.accounts) {
+      let status: HfStatus | undefined;
+      try {
+        status = await bridge.get(agentId);
+      } catch {
+        continue;
+      }
+      if (!status) continue;
+      if (this.lastStatus.get(agentId) === status) continue; // no change → no PUT
+
+      try {
+        const url = `${acct.guildBaseUrl.replace(/\/$/, '')}/agents/${encodeURIComponent(acct.fabricUserId)}/presence`;
+        const res = await fetch(url, {
+          method: 'PUT',
+          headers: {
+            'content-type': 'application/json',
+            'x-api-key': acct.fabricApiKey,
+          },
+          body: JSON.stringify({ status, source: 'hf-plugin' }),
+        });
+        if (res.ok) {
+          this.lastStatus.set(agentId, status);
+          this.logger.info(`fabric: presence-sync ${agentId} → ${status}`);
+        } else {
+          this.logger.warn(`fabric: presence-sync PUT ${agentId} failed: ${res.status}`);
+        }
+      } catch (err) {
+        this.logger.warn(`fabric: presence-sync PUT ${agentId} threw: ${String(err)}`);
+      }
+    }
+  }
+}