Problem

No standard forge-CLI affordance for checking out a PR. Today: figure out the remote branch name, `git fetch`, `git checkout` manually.

`gh pr checkout 123` is the bar.

Proposal

`anvil pr checkout `:

1. Resolves PR via existing `GET /pulls/:number` to get head_branch and head_repo.
2. `git fetch <head_branch>`
3. `git checkout <head_branch>` (or detached-head if branch already exists locally with different content — match `gh` behavior).

Acceptance criteria

• Single command runs fetch + checkout
• Handles cross-repo PRs (head_repo != base_repo)
• Errors gracefully outside a git repo
• Refuses to overwrite uncommitted changes

Origin

CLI gap audit.