BBC2-13 add bb pr checkout (same-repo PRs only)

[b2l]

Summary

• New bb pr checkout <id> that fetches the PR's source branch and either creates a local tracking branch or fast-forwards an existing one
• Fork-based PRs are detected (source vs destination repository.full_name) and refused with a clear error — full fork support is explicitly out of scope per the ticket
• Extends GitRunner with the write primitives fetchRef, checkoutExistingBranch, checkoutCreateTracking, mergeFastForwardOnly, plus the query hasLocalBranch. Failed writes throw a new GitError whose message carries git's stderr — the user sees git's own diagnostics (dirty working tree, diverged branches, missing remote ref) rather than a paraphrase.
• Extends PullRequestDetail with sourceRepositoryFullName / destinationRepositoryFullName so the fork check reads from the same typed GET /pullrequests/{id} response.

Test plan

• bun test — all 165 tests pass (added 10 integration tests against a real local bare remote covering fetch/hasLocalBranch/checkout/ff, happy + failure paths; added 2 backend mapping tests for the new fields)
• bun run lint clean
• bunx tsc --noEmit clean
• Manual: bb pr checkout --help shows correct help
• Manual: bb pr checkout (no id) errors with commander's required-arg message
• Manual: bb pr checkout abc / bb pr checkout 0 error with clear validation message

Manual test path

On any clone of a Bitbucket repo where you can see your own PRs:

1. Pick an open same-repo PR whose source branch does NOT exist locally. Run bb pr checkout <N>. Expect: Checked out pull request #N: <branch>. — you're switched to a new local branch tracking origin/<branch>.
2. Run bb pr checkout <N> again for the same PR. Expect: Checked out pull request #N: <branch> (fast-forwarded). — idempotent when nothing has changed remotely.
3. Make a local commit on that branch, then run bb pr checkout <N> once more. Expect a GitError with git's own "Not possible to fast-forward" output (we don't force).
4. On a dirty working tree (uncommitted changes overlapping with the target branch), run bb pr checkout <different-N>. Expect git's "Your local changes to the following files would be overwritten by checkout" error, surfaced verbatim.
5. If you can find a fork-based PR in your workspace, run bb pr checkout <N>. Expect: Pull request source repo 'other-ws/repo' differs from '<your-ws/repo>'. Fork-based PR checkout is not supported yet — open a follow-up if you need this.
6. bb --json pr checkout <N> emits {"id": ..., "branch": "...", "action": "created" | "fast-forwarded"}.

Closes BBC2-13.