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superseded ones collapse to one "Earlier plan revised" marker; step-attribution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

@@ -0,0 +1,142 @@
+package main
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestMessageQueue_FIFO(t *testing.T) {
+	q := newMessageQueue()
+	q.enqueue("s", "first")
+	q.enqueue("s", "second")
+	q.enqueue("s", "third")
+
+	want := []string{"first", "second", "third"}
+	for _, w := range want {
+		got, ok := q.dequeue("s")
+		if !ok || got != w {
+			t.Fatalf("dequeue = %q,%v want %q,true", got, ok, w)
+		}
+	}
+	if _, ok := q.dequeue("s"); ok {
+		t.Fatal("dequeue on drained queue should return ok=false")
+	}
+}
+
+func TestMessageQueue_RequeueFront(t *testing.T) {
+	q := newMessageQueue()
+	q.enqueue("s", "a")
+	q.enqueue("s", "b")
+	// Pop "a", then push it back to the front; "a" must come out before "b".
+	a, _ := q.dequeue("s")
+	q.requeueFront("s", a)
+	got, _ := q.dequeue("s")
+	if got != "a" {
+		t.Fatalf("after requeueFront, dequeue = %q want %q", got, "a")
+	}
+	got2, _ := q.dequeue("s")
+	if got2 != "b" {
+		t.Fatalf("next dequeue = %q want %q", got2, "b")
+	}
+}
+
+func TestMessageQueue_IsolatedPerSession(t *testing.T) {
+	q := newMessageQueue()
+	q.enqueue("s1", "one")
+	q.enqueue("s2", "two")
+	if got, _ := q.dequeue("s1"); got != "one" {
+		t.Fatalf("s1 = %q want one", got)
+	}
+	if got, _ := q.dequeue("s2"); got != "two" {
+		t.Fatalf("s2 = %q want two", got)
+	}
+	if q.peek("s1") != 0 || q.peek("s2") != 0 {
+		t.Fatal("both sessions should be drained")
+	}
+}
+
+func TestMessageQueue_Concurrent(t *testing.T) {
+	q := newMessageQueue()
+	const n = maxQueuedPerSession // stay under the cap so every enqueue lands
+	var wg sync.WaitGroup
+	for i := 0; i < n; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			q.enqueue("s", "m")
+		}(i)
+	}
+	wg.Wait()
+	if q.peek("s") != n {
+		t.Fatalf("peek = %d want %d (all enqueues must be counted)", q.peek("s"), n)
+	}
+	seen := 0
+	for {
+		if _, ok := q.dequeue("s"); !ok {
+			break
+		}
+		seen++
+	}
+	if seen != n {
+		t.Fatalf("drained %d want %d", seen, n)
+	}
+}
+
+func TestMessageQueue_CapsOverflow(t *testing.T) {
+	q := newMessageQueue()
+	for i := 0; i < maxQueuedPerSession; i++ {
+		if !q.enqueue("s", "m") {
+			t.Fatalf("enqueue #%d within cap should succeed", i)
+		}
+	}
+	if q.enqueue("s", "overflow") {
+		t.Fatal("enqueue past the cap should return false (dropped)")
+	}
+	if got := q.peek("s"); got != maxQueuedPerSession {
+		t.Fatalf("peek = %d want %d (overflow must not append)", got, maxQueuedPerSession)
+	}
+}
+
+// drainQueued on an empty queue must be a no-op: it returns immediately and
+// never touches the gate (so the session stays free for the next turn).
+func TestDrainQueued_NoOpOnEmpty(t *testing.T) {
+	a := &agent{gate: newTurnGate(), queue: newMessageQueue()}
+	a.drainQueued(context.Background(), "s")
+	if !a.gate.acquire("s", 0) {
+		t.Fatal("gate should be free after a no-op drain (drain must not hold it)")
+	}
+	a.gate.release("s")
+}
+
+// With a queued message but the gate held by another turn, drainQueued must
+// re-queue the message and return WITHOUT running a turn (no store/provider → a
+// real run would panic). This is the "never stack" property: a busy gate
+// defers to the holder's own release-drain.
+func TestDrainQueued_RequeuesWhenBusy(t *testing.T) {
+	prev := drainAcquireWait
+	drainAcquireWait = 10 * time.Millisecond
+	t.Cleanup(func() { drainAcquireWait = prev })
+
+	a := &agent{gate: newTurnGate(), queue: newMessageQueue()}
+	if !a.gate.acquire("s", 0) {
+		t.Fatal("precondition: hold the gate")
+	}
+	a.queue.enqueue("s", "queued-msg")
+
+	done := make(chan struct{})
+	go func() {
+		a.drainQueued(context.Background(), "s") // must not panic; must requeue
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-time.After(time.Second):
+		t.Fatal("drainQueued did not return promptly while the gate was busy")
+	}
+	if got := a.queue.peek("s"); got != 1 {
+		t.Fatalf("message should be re-queued while busy; peek = %d want 1", got)
+	}
+	a.gate.release("s")
+}