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superseded ones collapse to one "Earlier plan revised" marker; step-attribution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

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"log/slog"
+	"sync"
+)
+
+// maxQueuedPerSession caps a session's queue. A held turn plus unbounded
+// enqueues would grow memory without limit; an operator nudging a long
+// autonomous turn realistically queues only a handful, so a generous cap is
+// pure insurance. Overflow drops the newest enqueue and logs (the message is
+// already persisted in the DB by handleChat before enqueue, so it isn't lost
+// from the transcript — it just won't auto-run).
+const maxQueuedPerSession = 20
+
+// messageQueue holds operator messages that arrived while a turn was already
+// running for a session. Plan 2026-08-03 (F2): instead of rejecting the
+// operator's message with "Nomos is still finishing a previous step… send it
+// again", the message is queued and auto-run when the in-flight turn releases
+// the session's turn-gate permit.
+//
+// The queue only schedules WHEN a turn runs, not WHETHER the message is stored
+// — handleChat persists the user message before acquiring the gate, so a queued
+// message is already in the transcript; this just makes sure a turn eventually
+// acts on it.
+//
+// Draining is strictly one-at-a-time under the turn gate (see drainQueued in
+// main.go), so this cannot stack concurrent turns — the exact hazard the gate
+// itself exists to prevent. Background resumeSession callers never touch this
+// queue; they keep their non-blocking skip.
+type messageQueue struct {
+	mu    sync.Mutex
+	queue map[string][]string
+}
+
+func newMessageQueue() *messageQueue {
+	return &messageQueue{queue: map[string][]string{}}
+}
+
+// enqueue appends a message to the back of the session's FIFO. Returns false
+// (and logs) if the session is already at maxQueuedPerSession — the caller's
+// message is already persisted in the DB, so this only skips auto-running it.
+func (q *messageQueue) enqueue(sessionID, msg string) bool {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	if len(q.queue[sessionID]) >= maxQueuedPerSession {
+		slog.Warn("nomos: message queue full; dropping auto-run for operator message", "session", sessionID, "cap", maxQueuedPerSession)
+		return false
+	}
+	q.queue[sessionID] = append(q.queue[sessionID], msg)
+	return true
+}
+
+// dequeue pops the next message from the front of the session's FIFO. Returns
+// ok=false when empty.
+func (q *messageQueue) dequeue(sessionID string) (string, bool) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	xs := q.queue[sessionID]
+	if len(xs) == 0 {
+		return "", false
+	}
+	m := xs[0]
+	q.queue[sessionID] = xs[1:]
+	return m, true
+}
+
+// requeueFront pushes a message back to the front — used when a drainer popped
+// a message but lost the race for the gate to a live turn; that turn's own
+// release will drain it again.
+func (q *messageQueue) requeueFront(sessionID, msg string) {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	q.queue[sessionID] = append([]string{msg}, q.queue[sessionID]...)
+}
+
+// peek reports the queued depth for a session (test/diagnostic helper).
+func (q *messageQueue) peek(sessionID string) int {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+	return len(q.queue[sessionID])
+}