feat: undo last deletion — restore recently-removed worktree

[sauravpanda]

Problem

clean, prune, rm all permanently delete worktrees. If a user confirms by mistake (or the heuristic was wrong), the work is gone — they'd have to re-clone, re-create the branch, and lose any uncommitted local changes.

The existing snapshot command helps but requires prior action.

Suggested fix

• Before removing, automatically archive a lightweight snapshot of uncommitted changes (reuse the snapshot plumbing) to ~/.bonsai/trash/<timestamp>-<branch>/.
• Add bonsai undo to restore the most recent trashed worktree.
• Add bonsai trash list / bonsai trash empty for management.
• Trash entries auto-expire after N days (config-driven).

Optional: make this opt-in via --keep-history initially, opt-out later once stable.