feat(cli): Add direnv integration to load .envrc at startup

- Add init-direnv.ts to load direnv environment variables at CLI startup
- Search up directory tree for .envrc, stopping at git root
- Run `direnv export json` once and merge env vars into process.env
- Warn users if .envrc is blocked (need to run `direnv allow`)
- Add comprehensive tests (30 tests) for filesystem search, subprocess
  interactions, and full orchestration flow

Author: brandonkachen