Slice 12: inline command-result toasts

New <toast-host> overlay: components post 'toast' CustomEvents ({message, kind: success|error|info}) and it shows brief, auto-dismissing, bottom-right toasts (errors linger). <repo-menu> (git + coordination actions) and <pr-list> (create/merge) now post friendly success/error toasts instead of alert(); the activity feed still logs everything. Also adds the slice 11 CHANGELOG entry. Verified live: 'Checked for updates' toast shown.

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

web/templates/index.html

@@ -13,6 +13,7 @@
   <script type="module" src="/components/handoff-bar/handoff-bar.js"></script>
   <script type="module" src="/components/pr-list/pr-list.js"></script>
   <script type="module" src="/components/repo-menu/repo-menu.js"></script>
+  <script type="module" src="/components/toast-host/toast-host.js"></script>
   <style>
     header {
       display: flex;
@@ -55,5 +56,6 @@
     <activity-feed></activity-feed>
   </main>
   <repo-menu></repo-menu>
+  <toast-host></toast-host>
 </body>
 </html>