Fix/PR checkout branch name conflicts

[kirisame-wang]

Problem

Fixes #930

PR checkout fails when the PR's branch name conflicts with existing local or remote branches. This issue was introduced in #851 when adding fork PR support.

Solution

Use unique local branch names in pr-{number} format (e.g., pr-123) instead of the original branch name when checking out PRs.

Example:

# Before (fails if 'feat/my-feature' already exists)
git fetch origin pull/123/head:feat/my-feature

# After (always unique)
git fetch origin pull/123/head:pr-123

Changes

• Modified src/github/operations/branch.ts:
  • Store original branch name for logging only
  • Use pr-{number} as local branch name for checkout
  • Add detailed logging showing the branch name mapping

Benefits

• ✅ Guarantees uniqueness (PR numbers are unique per repo)
• ✅ Maintains fork PR support via GitHub's pull/{number}/head refs
• ✅ Works for both same-repo and fork PRs
• ✅ Simple and predictable naming convention

Testing

Existing tests cover the pr-{number} format (validated as hyphenated branch names).

Verification

✅ Tested successfully on private repository:

• Verified with a fork PR containing branch name test/claude-action-fork-fix
• Successfully checked out as pr-{number} format
• Logs confirm proper branch mapping and checkout process