fix(presence-sync): use /api prefix + Bearer guildAccessToken (not x-api-key)

Two layered bugs in the presence-sync loop, both causing every PUT to
fail forever in prod:

1. **Missing /api prefix.** URL was `${guildBaseUrl}/agents/<id>/presence`
   but the guild backend sets a global prefix 'api' in main.ts
   `setGlobalPrefix('api')`. Every other REST call in this plugin
   (channel.ts channels list, fabric-client.ts postMessage, canvas)
   already prepends /api/ — only presence-sync missed it. Returned 404
   "Cannot PUT /agents/...".

2. **Wrong auth scheme.** Plugin sent `x-api-key: <fabricApiKey>`, but
   the endpoint sits behind the global APP_GUARD = ApiKeyGuard, which
   actually expects `Authorization: Bearer <guildAccessToken>` (despite
   its name — confusing naming on the backend side). With /api added,
   error became 401 "missing bearer token". Confirmed by `docker exec
   fabric-backend-guild grep APP_GUARD /app/dist/app.module.js` and
   manual curl: Bearer guild token → 200 OK.

**Fix**

- presence-sync.ts: do agent-login on demand to obtain a fresh
  guildAccessToken, cache it per-agent for 13 min (under the 15-min
  JWT TTL), use it as Bearer for the PUT. 401 response invalidates
  the cache so the next tick re-logs-in. Pushes are gated on status
  changes (rare), so the login overhead is negligible.

- inbound.ts: firstGuildEndpointByAgent → firstGuildByAgent storing
  both endpoint and nodeId (presence-sync needs nodeId to pick the
  right token out of guildAccessTokens[]).

- index.ts: pass FabricClient to PresenceSync constructor.

**Verified in sim**

After restart, gateway log shows `fabric: presence-sync recruiter →
idle` (200 OK), zero failed PUTs, where previously it would log a 404
every ~5s per agent.

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

index.ts

@@ -116,7 +116,7 @@ export default defineChannelPluginEntry({
       // their fabricUserId + first guild endpoint populated).
       void inbound.start().then(() => {
         if (!inbound) return;
-        presence = new PresenceSync(api.logger);
+        presence = new PresenceSync(api.logger, client);
         presence.setAccounts(inbound.getPresenceAccounts());
         presence.start();
         api.logger.info(`fabric: presence-sync started for ${inbound.getPresenceAccounts().length} account(s)`);