1. branch_list: Fix is_head comparison
- head_name now keeps full ref (e.g., "refs/heads/main")
- Previously stripped to short name causing is_head to always be false
2. branch_get/branch_exists: Fix name resolution for branches with '/'
- Previously, "feature/testing" was assumed to be a remote branch
- Now tries both refs/heads/ and refs/remotes/ candidates
- Correctly handles local branches like "feature/testing"
3. sync_refs: Fix upstream reference format
- Was storing "refs/remotes/{}/branch" (broken pattern)
- Now properly queries git2 for actual upstream branch name
