When an agent branch is behind origin/<base>, branch-finish now rebases it automatically before integration instead of stopping with a manual sync error. This keeps long-lived agent branches up to date with main/dev without extra operator steps.

Constraint: Keep finish non-interactive and deterministic under behind-base drift
Rejected: Keep hard fail and require manual musafety sync each time | preserves friction and leaves branches stale by default
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: If rebase conflicts occur, keep abort/continue guidance explicit and avoid silent fallback merges
Tested: npm test (46/46), including updated finish auto-sync regression
Not-tested: Live conflict-heavy auto-sync scenario on a non-clean source branch (still blocked earlier by clean-worktree guard)