feat(FN-3888): fix dependency graph plugin ESM imports for Node16

Fixed ESM import resolution in the dependency graph plugin by repointing its tsconfig to Node16 and adding compatible import extensions, with verification tests covering the raw-node plugin entrypoint resolution path.

Fusion-Task-Id: FN-3888

packages/engine/src/__tests__/merger.test.ts

@@ -7655,8 +7655,9 @@ describe("aiMergeTask — in-merge verification fix", () => {
       [{ id: "FN-050", worktree: "/tmp/root/.worktrees/KB-050", column: "in-review" } as Task],
     );
     // Explicitly omit verificationFixRetries to test default behavior
+    const { verificationFixRetries: _omitVerificationFixRetries, ...settingsWithoutVerificationFixRetries } = DEFAULT_SETTINGS;
     (store.getSettings as ReturnType<typeof vi.fn>).mockResolvedValue({
-      ...DEFAULT_SETTINGS,
+      ...settingsWithoutVerificationFixRetries,
       testCommand: "vitest run",
       // verificationFixRetries is NOT set — should default to 2
     });