Smarter cbox cleanup: detect squash-merged PRs, ignore sandbox artifacts, scan all worktrees

Problem

The cbox cleanup command fails to reclaim most stale worktrees for three reasons: it cannot detect squash-merged PRs (only fast-forward merges), it treats .claude-sessions-sandbox/ and .claude/ directories as meaningful dirty state preventing cleanup, and it only scans cbox tmux sessions while ignoring worktrees created by Cursor or Codex. Over time, this leads to significant worktree sprawl — dozens of orphaned branches and worktrees consuming disk space and cluttering git worktree list output. During the pilot, worktree sprawl became a real developer friction point that required manual cleanup.

Context

• Squash-merged PRs detected via gh pr list --head <branch> --state merged.
• .claude-sessions-sandbox/ and .claude/ ignored when computing dirty state.
• Branches with 0 commits ahead of main classified as safe-to-clean.
• --worktrees flag scans all git worktrees, not just cbox sessions.
• Local branch deleted after worktree removal.

Possible Solutions

Plan

• Update _get_session_git_info() to check GitHub PR merge status.
• Filter .claude-sessions-sandbox/ and .claude/ from dirty check.
• Add commits-ahead check (git rev-list --count origin/main..HEAD).
• Add --worktrees flag to scan git worktree list in addition to cbox sessions.
• Delete local branch after worktree removal in cleanup execution.
• Add tests for the new classification logic.

Implementation Progress

• Directly reduces developer friction during pilot — worktree sprawl is a real pain point.
• No spillover expected; self-contained in cbox CLI.

Completion notes

Implemented in libs/cbox/cbox/cli.py. Key changes: - _get_session_git_info() now returns commits_ahead and squash_merged fields; uses three-tier merge detection (ancestor → commits-ahead → GH PR). - _IGNORED_DIRTY_PREFIXES tuple filters .claude-sessions-sandbox/, .claude/, .claude-sessions- from porcelain dirty check. - _remove_worktree() accepts branch= kwarg and runs git worktree prune before branch delete. - _list_git_worktrees() helper parses git worktree list --porcelain. - cleanup() accepts --worktrees flag, scans non-cbox worktrees, passes explicit branch name to _remove_worktree(). - 7 new test cases in libs/cbox/test_cleanup.py covering all features.

• GitHub issue: #412
• Affected code: libs/cbox/cbox/cli.py — _get_session_git_info(), cleanup()

Review Feedback

• [ ] Review cleared