feat(inbound): listen for backend-pushed channel.joined/left events

Companion to nav/Fabric.Backend.Guild#<TBD> which adds the server-side
emitToUser broadcast on channel membership changes. Before, the inbound
only learned about new channels via the 60s polling resync (worst-case
60s lag). Now the backend tells us directly so sub/unsub is realtime.

socket.on('channel.joined', evt) → join the socket.io room for evt.channelId
                                    and add to the local 'joined' set.
socket.on('channel.left',   evt) → leave + remove from 'joined'.

Both events are idempotent (`if (joined.has(id))` / `if (!joined.has(id))`)
so duplicate emits from server are safe. Polling resync still runs every
60s as a safety net for transient socket drops between emit and
reconnect, partial server failures, etc.

When backend lacks this support (older deployments), nothing breaks —
the event simply never fires and polling carries the load as before.

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

dist/fabric/src/inbound.js

@@ -154,6 +154,28 @@ export class FabricInbound {
                 joined.clear();
                 void syncChannels('initial');
             });
+            // Push-based membership events from the backend (companion to
+            // Fabric.Backend.Guild's RealtimeGateway.emitToUser). When the
+            // server tells us this user was added to / removed from a
+            // channel, we sub/unsub the socket.io room immediately — no
+            // 60s wait for the polling resync. Polling remains as a safety
+            // net for missed events.
+            socket.on('channel.joined', (evt) => {
+                const id = evt?.channelId;
+                if (!id || joined.has(id))
+                    return;
+                socket.emit('join_channel', { channelId: id });
+                joined.add(id);
+                this.log.info(`fabric: agent ${agentId} channel.joined push on ${g.nodeId}: ${id} (now ${joined.size})`);
+            });
+            socket.on('channel.left', (evt) => {
+                const id = evt?.channelId;
+                if (!id || !joined.has(id))
+                    return;
+                socket.emit('leave_channel', { channelId: id });
+                joined.delete(id);
+                this.log.info(`fabric: agent ${agentId} channel.left push on ${g.nodeId}: ${id} (now ${joined.size})`);
+            });
             const syncTimer = setInterval(() => void syncChannels('resync'), FabricInbound.CHANNEL_SYNC_INTERVAL_MS);
             this.channelSyncTimers.push(syncTimer);
             socket.on('message.created', (m) => {

src/inbound.ts

@@ -199,6 +199,26 @@ export class FabricInbound {
         joined.clear();
         void syncChannels('initial');
       });
+      // Push-based membership events from the backend (companion to
+      // Fabric.Backend.Guild's RealtimeGateway.emitToUser). When the
+      // server tells us this user was added to / removed from a
+      // channel, we sub/unsub the socket.io room immediately — no
+      // 60s wait for the polling resync. Polling remains as a safety
+      // net for missed events.
+      socket.on('channel.joined', (evt: { channelId?: string }) => {
+        const id = evt?.channelId;
+        if (!id || joined.has(id)) return;
+        socket.emit('join_channel', { channelId: id });
+        joined.add(id);
+        this.log.info(`fabric: agent ${agentId} channel.joined push on ${g.nodeId}: ${id} (now ${joined.size})`);
+      });
+      socket.on('channel.left', (evt: { channelId?: string }) => {
+        const id = evt?.channelId;
+        if (!id || !joined.has(id)) return;
+        socket.emit('leave_channel', { channelId: id });
+        joined.delete(id);
+        this.log.info(`fabric: agent ${agentId} channel.left push on ${g.nodeId}: ${id} (now ${joined.size})`);
+      });
       const syncTimer = setInterval(
         () => void syncChannels('resync'),
         FabricInbound.CHANNEL_SYNC_INTERVAL_MS,