Allow /plannotator-review <path> to review a specific worktree

## Summary

Allow `/plannotator-review` to accept a local path and use it as the review workspace/cwd.

Example:

```text
/plannotator-review .worktrees/my-feature
/plannotator-review /absolute/path/to/repo/.worktrees/my-feature
```

## Motivation

When working with agent-created git worktrees, the active chat/session cwd may be the main checkout while the implementation lives in a separate worktree. Today `/plannotator-review` opens review for the current session cwd, and passing a worktree path as a positional argument is ignored unless it is a URL.

This makes it awkward to review the correct worktree without starting a new agent/session from that worktree directory.

## Current behavior

The slash command supports PR URLs and flags such as `--git`, `--local`, and `--no-local`, but local non-URL positional args are not treated as a cwd/workspace target.

## Requested behavior

If the first positional argument is a local path, resolve it relative to the current cwd and pass it as the code review cwd.

Suggested behavior:

- `/plannotator-review ` reviews that local path/worktree.
- Existing `/plannotator-review ` behavior remains unchanged.
- Existing flags continue to work, e.g. `/plannotator-review .worktrees/my-feature --git`.
- Show a clear error if the path does not exist or is not a git/JJ workspace.

## Notes

The shared event API already supports `code-review` with a `cwd` payload, so this request is specifically about exposing that capability through the user-facing slash command.

Related issues:

- #196 covered worktree support generally.
- #295 asks for review triggering beyond the single manual slash command.
- #449 covers cwd/subdirectory behavior in code review.