feat(nomos): per-session turn serialization + chat reliability/UX fixes

The agent could run two turns for one session at once (a reconnect resumed
while the live turn was still going), and their interleaved tool calls
corrupted the activity panel, fabricated a confusing "parallel/nested"
sequence, and made tasks feel stuck/never-ending. Several UX gaps compounded it.

Turn serialization (F1):
- turnGate: at most one in-flight turn per session. Background resume paths
  (continuation worker, idle sweep, answer-question, /resume, reconnect)
  skip non-blocking when busy; the live chat path waits briefly then bails
  cleanly instead of stacking a second turn.
- resumeSession returns whether it ran; continueSession marks an execution
  "continued" only after a real run (review P0) so a busy-skip can't lose a
  finished-execution result. Idle nudge bumps only after delivery (P1).

Connection state (F2/F3, web):
- humanize/bucket raw errors ("model connection dropped..."); one surface
  per drop; a terminal task.status event clears stuck streaming/disconnected
  state and dismisses the connection toast. Reconnect no longer spawns turns.

Streaming where you look (F4, web):
- live command output in the global activity timeline and in the inline
  tool card (auto-opened, tail-pinned) -- not just the per-window rail.

Other (web): artifact/knowledge deep links (F5); step-first stable
"thinking" headline (F6); stable chat layout, no empty->content reflow (F7);
lazy event sync (P2.2); reconnect skips a terminal session (P2.1).

VERSION: 0.14.2 -> 0.15.0

cmd/nomos/continue.go

@@ -76,16 +76,20 @@ func (a *agent) processIdleSweep(ctx context.Context) {
 		s := s
 		if s.CompletionNudges == 0 {
 			safego.Go("nomos:idle-nudge:"+s.ID, func() {
-				if err := a.store.bumpCompletionNudge(ctx, s.ID); err != nil {
-					slog.Error("nomos: idle nudge bump failed", "session", s.ID, "error", err)
-					return
+				note := fmt.Sprintf("[System: this task ('%s') has been idle for %s with no complete_task call. "+
+					"If the goal is done (or can't be completed), call complete_task now with the outcome and a "+
+					"one-line summary. If you're still genuinely working through the plan, ignore this and continue.]",
+					s.Goal, idleTaskThreshold)
+				note = a.store.enrichResumeNote(ctx, s.ID, note)
+				// P1: only count the nudge if it actually delivered. resumeSession
+				// skips (returns false) when a turn is already active; bumping the
+				// counter anyway would make the next sweep auto-close a merely-busy
+				// session as "unanswered."
+				if a.resumeSession(ctx, s.ID, note) {
+					if err := a.store.bumpCompletionNudge(ctx, s.ID); err != nil {
+						slog.Error("nomos: idle nudge bump failed", "session", s.ID, "error", err)
+					}
 				}
-			note := fmt.Sprintf("[System: this task ('%s') has been idle for %s with no complete_task call. "+
-				"If the goal is done (or can't be completed), call complete_task now with the outcome and a "+
-				"one-line summary. If you're still genuinely working through the plan, ignore this and continue.]",
-				s.Goal, idleTaskThreshold)
-			note = a.store.enrichResumeNote(ctx, s.ID, note)
-			a.resumeSession(ctx, s.ID, note)
 			})
 			continue
 		}
@@ -163,7 +167,9 @@ func (a *agent) processContinuations(ctx context.Context) {
 				continue
 			}
 		}
-		a.store.markContinued(ctx, p.ExecID) // stamp first: a failure here must not cause a re-continue loop
+		// markContinued now happens inside continueSession, AFTER resumeSession
+		// actually runs (P0). Pre-marking here consumed the item even when
+		// resumeSession skipped on a busy session, losing the result.
 		safego.Go("nomos:continue-session:"+p.SessionID, func() { a.continueSession(ctx, p) })
 	}
 }
@@ -179,7 +185,18 @@ func (a *agent) processContinuations(ctx context.Context) {
 // something new to poll for.
 func (a *agent) continueSession(ctx context.Context, p pendingContinuation) {
 	slog.Info("nomos: auto-continuing session", "session", p.SessionID, "execution", p.ExecID, "status", p.Status)
-	a.resumeSession(ctx, p.SessionID, buildContinuationNote(p))
+	// P0 (plans/2026-08-03-nomos-chat-changes-review.md): mark the execution
+	// continued ONLY after the turn actually ran. resumeSession skips (returns
+	// false) when another turn is already active for this session; marking
+	// before that — as the old code did — consumed the item (continued_at set,
+	// never re-queued by pendingContinuations) and silently lost the result.
+	// On a skip, leave it pending so the next worker tick retries once the
+	// active turn frees the permit.
+	if !a.resumeSession(ctx, p.SessionID, buildContinuationNote(p)) {
+		slog.Info("nomos: continuation deferred — a turn is active; will retry next tick", "session", p.SessionID, "execution", p.ExecID)
+		return
+	}
+	a.store.markContinued(ctx, p.ExecID)
 }
 
 // resumeSession re-invokes the agent for a session with a system-injected note —
@@ -187,7 +204,26 @@ func (a *agent) continueSession(ctx context.Context, p pendingContinuation) {
 // (handleAnswerQuestion) — persisting progress LIVE (a placeholder row updated
 // in place as each tool call lands) so the frontend poller sees each step,
 // instead of total silence until the whole resume concludes.
-func (a *agent) resumeSession(ctx context.Context, sessionID, note string) {
+//
+// F1 (plan 2026-08-03): this is the single entry point for EVERY background
+// turn — the continuation worker, idle sweep, answer-question, /resume, and the
+// empty-message reconnect all funnel through here. It acquires the session's
+// turn permit non-blocking and SKIPS if a turn is already running. A duplicate
+// resume while a turn (live or background) is active is exactly the
+// interleaving that corrupted the activity panel and made tasks feel stuck.
+//
+// Returns whether the turn actually ran. Callers that mutate state before
+// resuming (the continuation worker's markContinued, the idle sweep's nudge
+// bump) MUST gate that mutation on a true return — otherwise a busy-skip leaves
+// the state changed but the work undone (lost continuation / false auto-close).
+// See plans/2026-08-03-nomos-chat-changes-review.md P0/P1.
+func (a *agent) resumeSession(ctx context.Context, sessionID, note string) bool {
+	if !a.gate.acquire(sessionID, 0) {
+		slog.Info("nomos: turn already active, skipping background resume", "session", sessionID)
+		return false
+	}
+	defer a.gate.release(sessionID)
+
 	placeholder, _ := json.Marshal(map[string]any{
 		"role": "assistant",
 		"text": "",
@@ -242,7 +278,7 @@ func (a *agent) resumeSession(ctx context.Context, sessionID, note string) {
 		if attempt > 0 {
 			select {
 			case <-cctx.Done():
-				return
+				return true // a turn ran on an earlier attempt; consume, don't re-loop
 			case <-time.After(time.Duration(2<<attempt) * time.Second): // 4s, 8s
 			}
 		}
@@ -314,9 +350,10 @@ func (a *agent) resumeSession(ctx context.Context, sessionID, note string) {
 			// No placeholder was inserted (rare), save directly.
 			a.store.saveMessage(context.Background(), sessionID, "assistant", body)
 		}
-		return // do not call persist() again — already persisted above
+		return true // do not call persist() again — already persisted above
 	}
 	persist() // final state — same row, updated one last time with the concluding text
+	return true
 }
 
 // buildContinuationNote frames the finished execution for the model: what