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>

cmd/server/main.go

@@ -121,6 +121,35 @@ func main() {
 		return c.JSON(http.StatusOK, svc.Activity(0))
 	})
 
+	// Graceful handoff (§8.3): the user requests a switch; Claude completes it.
+	e.GET("/api/switch", func(c echo.Context) error {
+		p, ok := svc.PendingSwitch()
+		if !ok {
+			return c.JSON(http.StatusOK, map[string]any{"pending": false})
+		}
+		return c.JSON(http.StatusOK, map[string]any{
+			"pending": true, "target": p.Target, "note": p.Note, "requestedAt": p.RequestedAt,
+		})
+	})
+	e.POST("/api/switch", func(c echo.Context) error {
+		var body struct {
+			Target string `json:"target"`
+			Note   string `json:"note"`
+		}
+		if err := c.Bind(&body); err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid body"})
+		}
+		p, err := svc.RequestSwitch(activity.ActorUser, body.Target, body.Note)
+		if err != nil {
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": err.Error()})
+		}
+		return c.JSON(http.StatusOK, map[string]any{"pending": true, "target": p.Target, "note": p.Note})
+	})
+	e.DELETE("/api/switch", func(c echo.Context) error {
+		svc.CancelSwitch(activity.ActorUser)
+		return c.JSON(http.StatusOK, map[string]any{"pending": false})
+	})
+
 	// SSE stream of activity events for live UI (§8.2).
 	e.GET("/events", func(c echo.Context) error {
 		w := c.Response()