open a file in the diff panel via clide ui open diff

Adds diff as a fourth ui.open target. The diff extension now retains an
app-scoped DiffController and subscribes to its builtin.diff/selection
channel: a selection reveals the diff tab and focuses the file, which
the view scrolls into view and highlights. Retaining the controller in
the extension (not the view) lets a focus survive the tab being
revealed/remounted, mirroring the ReaderNav viewers.

Closes T-233.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/src/daemon/ui_command.dart

@@ -26,11 +26,14 @@ typedef MessagePublisher = void Function(String publisher, String channel, Map<S
 
 /// The readers `ui.open` can target → (bus publisher id, payload key the
 /// reader reads its entry from). Matches each reader's `ReaderNav` dataKey
-/// (tickets/decisions key on `id`; markdown on `path`).
+/// (tickets/decisions key on `id`; markdown on `path`). `diff` is not a
+/// ReaderNav reader — it keys on `path` and its extension subscribes to the
+/// same `selection` channel to reveal its tab + focus the file (T-233).
 const Map<String, ({String publisher, String dataKey})> _readers = {
   'tickets': (publisher: 'builtin.tickets', dataKey: 'id'),
   'decisions': (publisher: 'builtin.decisions', dataKey: 'id'),
   'markdown': (publisher: 'builtin.markdown', dataKey: 'path'),
+  'diff': (publisher: 'builtin.diff', dataKey: 'path'),
 };
 
 /// Severities the toast verb accepts — mirrors `ToastSeverity` (kept as a