feat(FN-4067): add squash audit gate to merger with integration tests

Adds a squash-merge audit gate that verifies duplicate-cherry-pick commits and file-overlap losses after any squash merge into main, restoring the post-squash audit step to the merge workflow with test coverage for both the audit logic and the broader merger lifecycle.

Fusion-Task-Id: FN-4067

Fusion-Task-Lineage: 593aa917-5640-495b-b1ae-80c3eaa09d01

packages/engine/src/index.ts

@@ -31,6 +31,15 @@ export {
   type MergerOptions,
   type AutostashOrphanRecord,
 } from "./merger.js";
+export {
+  auditSquashMerge,
+  formatSquashAuditReport,
+  type SquashAuditFindings,
+  type SquashAuditFinding,
+  type SquashAuditDuplicateSubjectFinding,
+  type SquashAuditTouchedFileOverlapFinding,
+  type SquashAuditRecentMainCommit,
+} from "./merger-squash-audit.js";
 export { reviewStep, type ReviewType, type ReviewVerdict, type ReviewResult, type ReviewOptions } from "./reviewer.js";
 export { createFnAgent, promptWithFallback, describeModel, setHostExtensionPaths, getHostExtensionPaths, type AgentOptions, type AgentResult } from "./pi.js";