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/activity/activity.go

@@ -31,10 +31,21 @@ type Event struct {
 	Detail string    `json:"detail,omitempty"` // human-readable extra context
 }
 
+// PendingSwitch is a user's request for Claude to switch to another project.
+// It is the request half of the graceful handoff (§8.3) — Claude fulfils it at a
+// safe stopping point via AckSwitch.
+type PendingSwitch struct {
+	Target      string    `json:"target"`      // requested repo path
+	Note        string    `json:"note,omitempty"`
+	RequestedBy Actor     `json:"requestedBy"` // normally the user
+	RequestedAt time.Time `json:"requestedAt"`
+}
+
 // Feed is the concurrency-safe active-project + activity store with fan-out.
 type Feed struct {
 	mu        sync.RWMutex
-	active    string // active project path ("" = none)
+	active    string         // active project path ("" = none)
+	pending   *PendingSwitch // an outstanding switch request, if any
 	events    []Event
 	maxEvents int
 	nextID    int64
@@ -79,6 +90,65 @@ func (f *Feed) SetActiveProject(actor Actor, path string) (Event, bool) {
 	return ev, true
 }
 
+// RequestSwitch records a user's request for Claude to switch to target. The
+// latest request wins (it overwrites any outstanding one).
+func (f *Feed) RequestSwitch(actor Actor, target, note string) PendingSwitch {
+	f.mu.Lock()
+	p := PendingSwitch{Target: target, Note: note, RequestedBy: actor, RequestedAt: time.Now()}
+	f.pending = &p
+	ev, subs := f.appendLocked(actor, "switch-requested", target, note)
+	f.mu.Unlock()
+
+	publish(subs, ev)
+	return p
+}
+
+// PendingSwitch returns the outstanding switch request, if any.
+func (f *Feed) PendingSwitch() (PendingSwitch, bool) {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+	if f.pending == nil {
+		return PendingSwitch{}, false
+	}
+	return *f.pending, true
+}
+
+// AckSwitch completes a pending switch: it makes the requested target the active
+// project, clears the request, and records a "switch-completed" event carrying
+// Claude's summary of where it left the previous project. Returns false if there
+// was nothing pending.
+func (f *Feed) AckSwitch(actor Actor, summary string) (PendingSwitch, bool) {
+	f.mu.Lock()
+	if f.pending == nil {
+		f.mu.Unlock()
+		return PendingSwitch{}, false
+	}
+	p := *f.pending
+	f.pending = nil
+	f.active = p.Target
+	ev, subs := f.appendLocked(actor, "switch-completed", p.Target, summary)
+	f.mu.Unlock()
+
+	publish(subs, ev)
+	return p, true
+}
+
+// CancelSwitch clears a pending switch (e.g. the user changed their mind).
+func (f *Feed) CancelSwitch(actor Actor) (PendingSwitch, bool) {
+	f.mu.Lock()
+	if f.pending == nil {
+		f.mu.Unlock()
+		return PendingSwitch{}, false
+	}
+	p := *f.pending
+	f.pending = nil
+	ev, subs := f.appendLocked(actor, "switch-cancelled", p.Target, "")
+	f.mu.Unlock()
+
+	publish(subs, ev)
+	return p, true
+}
+
 // Record adds an arbitrary event to the feed.
 func (f *Feed) Record(actor Actor, kind, repo, detail string) Event {
 	f.mu.Lock()