Follow-up to bf4428c00 (FF-only ref advance). After the prevention fix
new orphans can't form, but pre-fix orphans like f6358ce4 on
fusion/fn-5419 still need a path back onto the integration branch.
Adds an `orphan-our-advance` classification to contamination recovery:
a "unique" foreign commit whose Fusion-Task-Id trailer points at a
`done` task AND that is unreachable from refs/heads/<integrationBranch>
is treated as a stranded merger output.
For these, the executor attempts a fast-forward rehome onto the
integration branch via advanceIntegrationBranchRef (which still enforces
the FF-only invariant). When successful, the orphan sha is added to the
existing `shasToDrop` set so the same recovery pass that drops
already-upstream/misrouted commits also drops the now-upstream orphan.
Non-FF orphans (diverged from current integration tip) are refused.
Doing a cherry-pick onto the integration branch from inside automated
recovery would introduce conflict-resolution surface that's too high
blast radius for a never-event recovery path. The refusal log line
includes the exact `git cherry-pick <sha>` command an operator can run
manually.
Two new GitMutationType audit events:
- merger:orphan-rehome-ff (successful FF rehome)
- merger:orphan-rehome-refused (non-FF, manual cherry-pick required)
Tests in merger-orphan-rehome.test.ts cover classification (orphan,
not-done, already-reachable, no-trailer) and the rehome operation
(FF success advances the ref + emits the audit event; non-FF refusal
emits the hint and leaves the ref untouched).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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