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>

components/repo-list/repo-list.md

@@ -21,6 +21,9 @@ component pattern the rest of the UI follows.
 - 2026-09-19: added a selected-item highlight — the clicked repo keeps an
   accent border/background (the item that `<repo-detail>` is showing). Caches the
   last `/api/repos` payload so re-selecting re-renders without a refetch.
+- 2026-09-20: selecting a repo now also `POST`s `/api/active-project` to make it
+  the active project (a user action, §8.2) — surfaced in `<activity-feed>` and
+  readable by Claude via `get_active_project`. Fire-and-forget.
 
 ## Notes / gotchas
 - Uses shared design tokens (`--git-*`, `--surface-*`, `--radius*`) for all