Pitch

Codex/opencode parity. The model just made an edit you don't want. Today you'd `/restore` against a snapshot — but that reverts the whole workspace state, not just the last edit. `/undo` is surgical: revert just the most recent `apply_patch` / `edit_file` / `write_file` tool result.

Implementation sketch

1. Every successful patch tool call records a per-file backup (we already have `snapshot::repo` + the workspace-side git mirror).
2. `/undo` walks the most recent tool calls back, picks the first patching call, restores the affected files from that snapshot.
3. Status toast: "Reverted apply_patch from turn N".
4. `/undo 3` undoes the 3 most recent patching calls in reverse order.

Acceptance

• `/undo` reverts the last patching tool call.
• `/undo N` reverts the last N patching calls.
• No-op with toast when there's nothing to undo.
• Tests cover apply_patch, edit_file, write_file paths.