Fixes daemon and bd sync to honor BEADS_SYNC_BRANCH environment variable
as documented in PROTECTED_BRANCHES.md for CI/CD temporary overrides.
Changes:
- Updated internal/syncbranch.Get() to prioritize env var over DB config
- Both daemon sync and bd sync CLI now use syncbranch.Get()
- Added comprehensive tests for env var override behavior
- Validates branch names using git-style rules
This enables CI/CD workflows to override sync branch per-job without
mutating database config.
Based on PR #364 by Charles P. Cross <cpdata@users.noreply.github.com>
Co-authored-by: Charles P. Cross <cpdata@users.noreply.github.com>
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