plan-approve-once policy + cooler empty states + remove graph header

Backend:
- proposePlan sets plan window in autonomy_settings (nomos:plan:<session>)
- run handler checks plan window — auto-executes config_mutation commands
  within plan without per-action approval
- planWindowActive function in server.go
- Plan window cleaned up on completeTask (already covered by LIKE '%:' || )

Frontend:
- Removed 'Session graph' header bar
- Cooler empty states: Plan shows animated dots + 'Awaiting plan…',
  Activity shows pulsing dots + 'Waiting for activity…'

cmd/nomos/store.go

@@ -464,6 +464,15 @@ func (s *store) proposePlan(ctx context.Context, sessionID string, steps []planS
 	// rather than replace it (mirrors the mid-flight append above).
 	_ = observability.Event(ctx, sqlcgen.New(s.pool), "plan.proposed", s.taskEntityPtr(ctx, sessionID),
 		"info", "nomos", sessionID, map[string]any{"steps": out, "appended": anyStarted})
+	// Record a plan-proposed window so the `run` handler knows a plan is
+	// pending approval and can skip per-action approval for config_mutation
+	// commands within the plan. Transitions to 'active' when the operator
+	// approves (chat-assent or button). The window key is session-scoped;
+	// one agent serves all sessions on this nomos instance.
+	s.pool.Exec(ctx,
+		`INSERT INTO autonomy_settings (key, value) VALUES ($1, 'proposed')
+		 ON CONFLICT (key) DO UPDATE SET value = 'proposed'`,
+		"nomos:plan:"+sessionID)
 	return out, nil
 }