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superseded ones collapse to one "Earlier plan revised" marker; step-attribution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

web/src/lib/stores/chat.ts

@@ -72,6 +72,21 @@ function mid(): string {
   return crypto.randomUUID()
 }
 
+// dropOptimisticAssistantBubble removes the trailing empty assistant message
+// that sendSessionMessage/startTask optimistically append — used when a turn is
+// QUEUED behind an in-flight one (plan 2026-08-03 F2): no live assistant stream
+// is attached, so the empty placeholder must go (otherwise it lingers as a
+// blank bubble). Shared so the guard can't drift between the two call sites.
+function dropOptimisticAssistantBubble(messages: Writable<ChatMessage[]>): void {
+  messages.update((ms) => {
+    const last = ms[ms.length - 1]
+    if (last && last.role === 'assistant' && last.text === '' && last.tools.length === 0) {
+      return ms.slice(0, -1)
+    }
+    return ms
+  })
+}
+
 export const messages = writable<ChatMessage[]>([])
 export const streaming = writable(false)
 export const connectionState = writable<'connected' | 'disconnected' | 'reconnecting'>('connected')
@@ -657,6 +672,16 @@ export function sendSessionMessage(sessionId: string, text: string) {
     sessionId,
     (ev: ChatEvent) => {
       if (ev.type === 'session') return // sessionId is already known for a window
+      if (ev.type === 'queued') {
+        // This message was queued behind an in-flight turn (plan 2026-08-03
+        // F2): no assistant stream is attached to this response. Drop the
+        // optimistic empty assistant bubble so the user message is the last
+        // thing on screen — the thread then shows a "Queued" hint while the
+        // session is working, and the poller surfaces the queued turn's
+        // result once it runs server-side.
+        dropOptimisticAssistantBubble(chat.messages)
+        return
+      }
       if (ev.type === 'tool_use') {
         const tr: ToolCallResult = {
           type: 'tool_use',
@@ -791,6 +816,13 @@ export function startTask(text: string, onSession: (sessionId: string) => void):
   function apply(ev: ChatEvent) {
     const c = chat
     if (!c || !sessionId) return
+    if (ev.type === 'queued') {
+      // Defensive: a brand-new task won't normally queue (its session has no
+      // in-flight turn), but handle it symmetrically with sendSessionMessage —
+      // drop the optimistic empty assistant bubble. See plan 2026-08-03 F2.
+      dropOptimisticAssistantBubble(c.messages)
+      return
+    }
     if (ev.type === 'tool_use') {
       const tr: ToolCallResult = {
         type: 'tool_use',