Slice 3: activity feed and active project (SSE + MCP tools)

Add internal/activity (thread-safe active project + bounded event feed with subscriber fan-out). Service exposes active-project/activity methods and takes the feed. New MCP tools get_active_project/set_active_project/get_activity and HTTP endpoints GET/POST /api/active-project, /api/activity, and /events (SSE). New <activity-feed> component updates live via EventSource; <repo-list> sets the active project on selection. Verified end-to-end in the running app: user actions push live events and set the active project (actor=user), all readable by Claude over MCP.

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

web/templates/help.html

@@ -53,6 +53,15 @@
       most recent commits.
     </p>
 
+    <h2>Active project &amp; activity</h2>
+    <p>
+      Clicking a repository also makes it your <strong>active project</strong> —
+      the one you're currently focused on — shown in the activity panel at the
+      bottom. That panel also lists recent actions by both you and Claude, updating
+      live. When GitManager is connected to Claude, Claude can see your active
+      project and this activity, so you stay on the same page.
+    </p>
+
     <h2>Background refresh</h2>
     <p>
       The dashboard refreshes on its own. By default it does <em>not</em> reach