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_publisher.dart

@@ -22,6 +22,10 @@ abstract final class ClaudeConversation {
   /// Channel for the lead (or single) Claude pane's conversation.
   static const leadChannel = 'conversation';
 
+  /// Per-session channel, keyed by the Claude session id (T-146) so
+  /// concurrent panes in one workspace don't cross-talk.
+  static String sessionChannel(String sessionId) => 'conversation/$sessionId';
+
   /// Channel for a teammate's conversation (team work, T-139/T-140).
   static String teammateChannel(String agentId) => 'conversation/$agentId';