1. Init: cd to ~/.kugetsu-worktrees before creating base session
This keeps all worktrees inside a predictable directory structure
and avoids external_directory permission issues
2. Fork context: Add kugetsu_get_fork_context() that provides:
- Important working rules (stop on error, don't pivot)
- Repository configuration from repos.json
- Environment file location info
3. Fork message: Prepend context to user message when forking session
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