Slice 4: graceful project handoff (pending switch + ack)

Add pending-switch coordination to internal/activity (RequestSwitch/PendingSwitch/AckSwitch/CancelSwitch); AckSwitch atomically sets the active project and records switch-completed with Claude's summary. New MCP tools get_pending_switch and ack_switch (request is user-only via HTTP). HTTP GET/POST/DELETE /api/switch. New <handoff-bar> component (ask/waiting/cancel/completed) over SSE; activity-feed reflects switch-completed. Test covers request->ack->clear. Verified live: request/waiting/cancel over SSE.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/service/service.go

@@ -86,3 +86,32 @@ func (s *Service) Activity(limit int) []activity.Event {
 func (s *Service) SubscribeActivity() (<-chan activity.Event, func()) {
 	return s.feed.Subscribe()
 }
+
+// --- Graceful project handoff (§8.3) ---------------------------------------
+
+// RequestSwitch records a user's request for Claude to switch to target. The
+// target must be an indexed repository.
+func (s *Service) RequestSwitch(actor activity.Actor, target, note string) (activity.PendingSwitch, error) {
+	target = filepath.Clean(target)
+	if _, ok := s.index.Get(target); !ok {
+		return activity.PendingSwitch{}, fmt.Errorf("unknown repository %q", target)
+	}
+	return s.feed.RequestSwitch(actor, target, note), nil
+}
+
+// PendingSwitch returns the outstanding switch request, if any.
+func (s *Service) PendingSwitch() (activity.PendingSwitch, bool) {
+	return s.feed.PendingSwitch()
+}
+
+// AckSwitch completes the pending handoff on Claude's behalf: sets the active
+// project to the requested target and records Claude's summary. Returns false if
+// nothing was pending.
+func (s *Service) AckSwitch(summary string) (activity.PendingSwitch, bool) {
+	return s.feed.AckSwitch(activity.ActorClaude, summary)
+}
+
+// CancelSwitch clears a pending switch request.
+func (s *Service) CancelSwitch(actor activity.Actor) (activity.PendingSwitch, bool) {
+	return s.feed.CancelSwitch(actor)
+}