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superseded ones collapse to one "Earlier plan revised" marker; step-attribution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/agent.go

@@ -60,6 +60,10 @@ type agent struct {
 	// gate serializes turns per session (at most one in-flight turn per
 	// sessionID). See turngate.go and plan 2026-08-03 F1.
 	gate *turnGate
+	// queue holds operator messages that arrived while a turn was already
+	// running; they are auto-run when the gate frees (plan 2026-08-03 F2).
+	// See messagequeue.go.
+	queue *messageQueue
 }
 
 func newAgent(ctx context.Context, clients *mcpClientPool, st *store, agentSlug string) (*agent, error) {
@@ -121,6 +125,7 @@ func newAgent(ctx context.Context, clients *mcpClientPool, st *store, agentSlug
 		apiToken:   os.Getenv("OIKOS_MCP_BEARER_TOKEN"),
 		httpClient: &http.Client{Timeout: 15 * time.Second},
 		gate:       newTurnGate(),
+		queue:      newMessageQueue(),
 	}, nil
 }