fix(session): merge-time fixes for the dead-pane resume pin (#467)

- test/setup.ts strips CLAUDE_CONFIG_DIR (pinned in test-env-isolation), so
  transcript-fixture tests such as session-custom-model-restart no longer go
  red on a machine that exports it for a separate Claude account (#255).
- The vanished-tmux-session branch of _setupOrAttachMuxSession() relaunches
  the CLI through createSession() just like a failed respawn, so it now takes
  the same resume pin. A genuinely new session is unaffected.
- After a dead-pane respawn of a fallback-chain CLI, _claudeSessionId names
  the conversation the walk actually pinned instead of the chain tail, which
  the walk may have passed over for lack of a transcript.
- _claudeConfigDir() trims the override like claudeProjectsDir() does.
- The remote-reattach test is labelled as documentation, since the pin
  builder's own remote guard would make it pass either way.
- CLAUDE.md: the create-path pin persists through toState() as
  resumeSessionId, and the end of the walk adds no pin rather than clearing
  the launch seed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

test/test-env-isolation.test.ts

@@ -34,6 +34,7 @@ const STRIPPED_ENV_VARS: Array<[name: string, why: string]> = [
   ['CODEMAN_INSTANCE', 'moves the data dir to ~/.codeman-<name> and the tmux socket to codeman-<name>'],
   ['CODEMAN_DATA_DIR', 'ABSOLUTE override: bypasses the temp HOME and points the suite at a real data dir'],
   ['CODEMAN_TMUX_SOCKET', 'renames the socket resolveTmuxSocketName() returns'],
+  ['CLAUDE_CONFIG_DIR', 'relocates the Claude tree, so transcript fixtures under the temp HOME read as missing'],
 ];
 
 const SETUP_SOURCE = readFileSync(fileURLToPath(new URL('./setup.ts', import.meta.url)), 'utf-8');