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>

web/templates/index.html

@@ -10,6 +10,7 @@
   <script type="module" src="/components/repo-list/repo-list.js"></script>
   <script type="module" src="/components/repo-detail/repo-detail.js"></script>
   <script type="module" src="/components/activity-feed/activity-feed.js"></script>
+  <script type="module" src="/components/handoff-bar/handoff-bar.js"></script>
   <style>
     header {
       display: flex;
@@ -40,6 +41,7 @@
     <nav><a href="/help">Help</a></nav>
   </header>
   <main>
+    <handoff-bar></handoff-bar>
     <div class="cols">
       <repo-list></repo-list>
       <repo-detail></repo-detail>