fix(daemon): check sync-branch upstream for jj/jujutsu compatibility

[phredrick42]

Summary

• When sync-branch is configured, daemon now checks that branch's upstream instead of current HEAD's upstream
• Adds gitBranchHasUpstream(branch) to check a specific branch's upstream tracking, independent of HEAD state
• Fixes --auto-push compatibility with jj/jujutsu which always operates in detached HEAD mode

Test plan

• [x] Added unit tests for gitBranchHasUpstream() covering:
  • Branch with upstream returns true
  • Non-existent branch returns false
  • Works with detached HEAD (jj scenario)
  • Branch without upstream returns false
• [x] Manual E2E test: verified old daemon fails and new daemon succeeds with jj colocated repo in detached HEAD state

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