fix: move worktrees to app data

[Dimillian]

Summary

• move new worktrees under app data instead of writing .codex-worktrees into repos
• stop mutating user .gitignore when creating worktrees
• keep legacy .codexmonitor as-is (no migration; existing sessions still load)

Behavior changes

• new worktrees are stored in app data: worktrees/
• repositories are no longer modified during worktree creation
• existing .codexmonitor state is still read; otherwise Codex uses its default home

Testing

• npm run typecheck
• npm run tauri dev