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>

test/builtin/claude/transcript_reader_test.dart

@@ -718,6 +718,31 @@ void main() {
         'reply number 199',
       );
     });
+
+    test('explicit file: tails that exact file, ignoring newest-discovery', () async {
+      // A teammate reader (T-139) points at one fixed subagent file rather
+      // than the newest .jsonl in the munged dir.
+      final dir = await Directory.systemTemp.createTemp('explicit_file_');
+      addTearDown(() => dir.delete(recursive: true));
+      final target = File('${dir.path}/agent-abc.jsonl');
+      writeLines(target, [assistantText('a1', 'from the explicit file')]);
+
+      final reader = TranscriptReader(
+        '/unused/workspace',
+        projectsBase: '/nonexistent',
+        pollInterval: const Duration(milliseconds: 20),
+        file: target.path,
+      );
+      final collected = <ConversationItem>[];
+      final sub = reader.stream.listen(collected.add);
+
+      await pumpUntil(() => collected.isNotEmpty);
+
+      await sub.cancel();
+      await reader.dispose();
+
+      expect(collected.whereType<AssistantTextMessage>().single.text, 'from the explicit file');
+    });
   });
 }