Adopts the documentation and integration test additions from the
Fusion executor's parallel work on FN-4333 (sleek-oak worktree). The
core hot-fix already landed in ccd718acd.
- AGENTS.md: update the post-squash audit paragraph to describe the
three modes and the verified-tree short-circuit.
- docs/settings-reference.md: add postMergeAuditMode row.
- packages/engine/src/__tests__/merger-merge-lifecycle.test.ts: add
integration tests covering postMergeAuditMode=off (skips audit) and
postMergeAuditMode=warn (continues despite dirty findings).
Fusion-Task-Id: FN-4333
1777a34c7a