bind each Claude pane to its own session id (T-146)

A regression from T-137: every pane rendered the newest .jsonl in the
workspace dir, so concurrent sessions collided — a secondary tab showed
the primary's conversation. Each pane now spawns claude with its own
--session-id (a transcript is named <session-id>.jsonl), tails that
exact file via TranscriptReader's file: param, and uses a per-session
MessageBus channel so controllers don't cross-talk.

The primary's id is deterministic from its session name (stable → it
resumes across restarts, like /resume off the same history file);
secondaries get a fresh random id so a clean session is always available.
The reader now waits for the bound file to appear rather than throwing.

Migration: an existing tmux session created before this (no --session-id,
claude chose its own id) must be killed once (claude.kill-all-sessions)
so the next spawn binds the controlled id.

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

lib/builtin/claude/src/transcript_reader.dart

@@ -294,10 +294,13 @@ class TranscriptReader {
   }
 
   Future<void> _tick(StreamController<ConversationItem> controller) async {
-    // A teammate reader tails one fixed file; otherwise discover the
+    // A pane/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;
+    // An explicit file may not exist yet (claude writes it shortly after
+    // spawn) — wait for it rather than throwing in the poll loop.
+    if (!await File(newest).exists()) return;
 
     if (newest != _currentPath) {
       // New session file. Start from the recent tail rather than byte 0: