fix(cli): avoid app-local agent shims

[devswha]

Summary

• remove app-local node_modules/.bin directories from external-agent executable lookup
• resolve the first executable user installation to an absolute path before starting tmux
• retain the existing bare-command failure behavior when no executable can be resolved

Root cause

npm run dev prepends ChatMux's own node_modules/.bin to PATH. A new Codex tmux therefore launched the repository SDK dependency (0.141.0) instead of the user's installed Codex CLI (0.144.6). The older binary could not run the configured gpt-5.6-sol model. This was reproduced twice and confirmed from the live process tree.

Verification

• external session service tests (28/28)
• npm run typecheck
• npm run lint
• npm run build

[devswha]

E2E QA evidence (2026-07-22): a newly created Codex tmux showed global Codex 0.144.6 instead of the repository-local 0.141.0 shim, then completed the requested file edit and validation commands.