Fresh clones with sync-branch configured in .beads/config.yaml would show
.beads/issues.jsonl as modified in git status because the git index flags
(skip-worktree, assume-unchanged) are local-only and don't transfer via clone.
This fix ensures bd init sets these flags in two scenarios:
1. `bd init --branch <name>` - when user explicitly sets sync branch
2. `bd init` on cloned repo - when sync-branch already exists in config.yaml
Added SetSyncBranchGitignoreFlags() helper and two tests for coverage.