feat(nomos): chat working-visibility, message queue, generation-aware timeline

Make background/long/desynced turns visible and queueable, fixing the four
symptoms that survived the v0.15.0 chat reliability pass.

F1 - status-driven working signal (workspace.ts taskWorking/currentWorking =
streaming OR status in {planning,executing}). Drives the chat trace, indicator,
and activity spinner so a turn with no live stream (background resume, a dropped
SSE, an idle-close mid long turn) still looks alive.

F2 - operator messages sent during an in-flight turn are now QUEUED and
auto-run when the gate frees, replacing the "still finishing a previous step...
send it again" rejection. Per-session in-memory FIFO (messagequeue.go, capped at
20) drained one-at-a-time under the turn gate; a `queued` SSE event drives a
"Queued" hint. drainQueued releases via a per-iteration deferred closure so a
runChatTurn panic can't deadlock the session's gate.

F3 - SSE keepalive (12s `:keepalive` comment) in handleChat so 20-40s
inter-iteration gaps no longer trip a proxy/browser idle close (the desync root
cause). All SSE writes serialized through one mutex.

F4 - generation-aware activity timeline (only the last propose_plan renders;
superseded ones collapse to one "Earlier plan revised" marker; step-attribution
follows only the current generation) + debounced plan refetch on lifecycle
events so a missed plan.proposed self-heals.

Verified against the last session (23da10db: 6m33s turn, operator "status"
deferred at 19:48:05). go test ./cmd/nomos/ green (new messagequeue tests);
web vitest 72/72 (new F4 generation tests); vite build clean.

VERSION: 0.16.0 -> 0.17.0

cmd/nomos/continue.go

@@ -222,7 +222,13 @@ func (a *agent) resumeSession(ctx context.Context, sessionID, note string) bool
 		slog.Info("nomos: turn already active, skipping background resume", "session", sessionID)
 		return false
 	}
-	defer a.gate.release(sessionID)
+	// Release the gate, then drain any operator message that was queued while
+	// this background turn ran (plan 2026-08-03 F2). Queued messages are run as
+	// real user turns server-side; resumeSession itself never enqueues.
+	defer func() {
+		a.gate.release(sessionID)
+		safego.Go("nomos:drain:"+sessionID, func() { a.drainQueued(context.Background(), sessionID) })
+	}()
 
 	placeholder, _ := json.Marshal(map[string]any{
 		"role": "assistant",