add per-session status strip: model / permission-mode / context (T-145)

The transcript reader now also extracts a SessionStatus — current model
(assistant message.model), permission mode (the permission-mode records,
previously skipped), and context-window tokens (message.usage input +
cache-read + cache-creation) — and emits it on a statusStream, merging
deltas so it only fires on change. All CC-internals parsing stays in the
drift-contained reader (D-75).

The Claude pane renders this as a thin strip above the conversation
(model · permission-mode · context). Context is shown as a token count,
not a percentage: the transcript carries usage but not the model's window
limit, and the model id doesn't encode the 1M vs 200k tier.

Lead pane done; teammate-tile mirror and the sidebar (T-141) consume the
same status next.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/transcript_publisher.dart

@@ -56,6 +56,10 @@ class TranscriptPublisher {
   final String channel;
   late final StreamSubscription<ConversationItem> _sub;
 
+  /// Live session status (model / permission-mode / context) from the
+  /// underlying reader — passed through for the status strip (T-145).
+  Stream<SessionStatus> get statusStream => _reader.statusStream;
+
   /// Stops publishing and tears down the underlying reader.
   Future<void> dispose() async {
     await _sub.cancel();