feat: add interactive stale worktree cleanup with wtk clean

[nettee]

What to build

Add wtk clean, an interactive command for cleaning up leftover/stale worktrees. The command should inspect candidate worktrees, explain why each one appears safe or risky to remove, show a confidence level for cleanup, and support confirmed batch cleanup.

The goal is to make cleanup safer and more transparent than manually deleting directories or running low-level Git commands.

Acceptance criteria

• wtk clean discovers leftover/stale worktree cleanup candidates.
• Each candidate includes a human-readable explanation and a cleanup confidence level, such as high/medium/low.
• The command is interactive by default and requires explicit confirmation before deleting/pruning anything.
• Users can select multiple candidates and clean them in one confirmed batch.
• Low-confidence or ambiguous candidates are clearly marked and are not silently cleaned.
• The command fails fast with a clear error if required repository/worktree state cannot be determined.
• Tests cover discovery, confidence classification, confirmation flow, cancellation, and batch cleanup behavior.

Blocked by

None - can start immediately