fix(branch): avoid fork PR fetch collision with checked-out branch

[syf2211]

Summary

Fixes fork PR checkout failures in tag-mode workflows when the fork head branch name matches the branch already checked out on the runner (e.g. main).

Motivation

setupBranch fetched cross-repo PR heads with pull/<N>/head:<headRefName>, which writes into refs/heads/<headRefName>. If that branch is currently checked out, git refuses with:

fatal: refusing to fetch into branch 'refs/heads/main' checked out at '...'

This breaks issue_comment / tag-mode runs on fork PRs opened from the fork's default branch.

Fixes #1423

Changes

• Fetch fork PR heads into a namespaced remote ref: refs/remotes/pull/<N>/head
• Use force fetch (+) so reruns can update the ref safely
• When the fork head name collides with the current branch, check out the PR head in detached mode
• Otherwise create/update a local branch from the fetched PR head so git push keeps working for differently named fork heads
• Add unit tests for the new checkout helpers

Tests

• bun test test/fork-pr-branch-checkout.test.ts — pass
• bun run typecheck — pass

Notes

Detached checkout is only used for the collision case described in #1423. Non-colliding fork PRs still get a local branch at headRefName.