add tmux team observer + member lifecycle events (T-139, D-75)

team_observer.dart is the single drift-containment point for Claude
Code's experimental tmux team mode. It discovers the active team for a
workspace (~/.claude/teams/<team>/config.json, matched by member cwd),
polls `tmux -L clide list-panes -a`, and correlates live panes with the
config's tmuxPaneId to emit TeamMemberBorn / TeamMemberDied — identity
(name, agentType, model, colour, pane) comes from the config, so it's
reliable regardless of transcript drift.

Each teammate's subagent transcript is resolved best-effort and streamed
on a per-agent MessageBus channel via TranscriptPublisher (TranscriptReader
gains an explicit `file:` for this). The config<->transcript join is the
fragile part: no shared key, so it uses a sibling .meta.json agentType
when present, else zips members-by-joinedAt against files-by-mtime. This
join needs validation against a live team run.

App wiring + visible surfacing land with the teammate tiles (T-140).

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

lib/builtin/claude/src/transcript_reader.dart

@@ -187,10 +187,12 @@ class TranscriptReader {
     void Function(String)? onWarn,
     String? projectsBase,
     int? initialTailBytes,
+    String? file,
   })  : _pollInterval = pollInterval,
         _onWarn = onWarn ?? _defaultWarn,
         _projectsBase = projectsBase ?? _defaultProjectsBase(),
-        _initialTailBytes = initialTailBytes ?? _defaultInitialTailBytes;
+        _initialTailBytes = initialTailBytes ?? _defaultInitialTailBytes,
+        _explicitFile = file;
 
   final String workspacePath;
   final Duration _pollInterval;
@@ -205,6 +207,11 @@ class TranscriptReader {
   /// temp directory instead of the user's home.
   final String _projectsBase;
 
+  /// When set, tail this exact file instead of discovering the newest
+  /// `.jsonl` in the munged dir. Used for teammate subagent transcripts
+  /// (T-139), whose path the team observer resolves explicitly.
+  final String? _explicitFile;
+
   static String _defaultProjectsBase() {
     final home = Platform.environment['HOME'] ?? '';
     return home.isNotEmpty ? '$home/.claude/projects' : '.claude/projects';
@@ -287,10 +294,9 @@ class TranscriptReader {
   }
 
   Future<void> _tick(StreamController<ConversationItem> controller) async {
-    final dir = _mungedDir();
-
-    // Discover or refresh the active session file.
-    final newest = await _newestJsonl(dir);
+    // A teammate reader tails one fixed file; otherwise discover the
+    // newest session `.jsonl` in the munged dir.
+    final newest = _explicitFile ?? await _newestJsonl(_mungedDir());
     if (newest == null) return;
 
     if (newest != _currentPath) {