FN-8009: preserve approved plans through prompt hygiene

Keep manually approved plans idempotent when deterministic prompt hygiene is applied.

- Document normalized fingerprint comparison at the approval gate
- Cover approval reuse after Original Description and Frontend UX injection

Files changed:
 packages/engine/src/__tests__/triage.test.ts | 37 ++++++++++++++++++++++++++++
 packages/engine/src/triage.ts                |  8 ++++++
 2 files changed, 45 insertions(+)

Fusion-Task-Id: FN-8009

Fusion-Task-Lineage: 8474bc13-61e0-421a-8e99-080f99382285

Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>

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

@@ -3206,6 +3206,43 @@ describe("requirePlanApproval setting", () => {
       );
     });
 
+    /*
+     * FNXC:PlanApproval 2026-07-15-21:10:
+     * FN-8009 requires the as-approved comparison to cover both deterministic hygiene
+     * mutations. A legacy operator-approved plan may lack both its original request and
+     * the frontend checklist; these injections must not turn an unchanged plan into a
+     * new manual approval request.
+     */
+    it("auto-approves a pre-hygiene plan after Original Description and Frontend UX injection", async () => {
+      const frontendPlan = "# Task: FN-FRONTEND-HYGIENE - Frontend plan\n\n## Mission\n\nUpdate the dashboard.\n\n## File Scope\n\n- `packages/dashboard/app/TaskCard.tsx`\n";
+      const task = createTriageTask({
+        id: "FN-FRONTEND-HYGIENE",
+        description: "The original dashboard request is absent from this planner output.",
+        status: "planning",
+        approvedPlanFingerprint: computePlanApprovalFingerprint(frontendPlan),
+      } as Partial<Task>);
+      const store = createMockStore({
+        getTask: vi.fn().mockResolvedValue(task),
+      } as Partial<TaskStore>);
+      await mkdir(join(rootDir, ".fusion", "tasks", task.id), { recursive: true });
+      await writeFile(join(rootDir, ".fusion", "tasks", task.id, "PROMPT.md"), frontendPlan);
+      const processor = new TriageProcessor(store, rootDir);
+
+      await (processor as unknown as {
+        finalizeApprovedTask(task: Task, writtenInput: string, settings: Settings): Promise<void>;
+      }).finalizeApprovedTask(
+        task,
+        frontendPlan,
+        { requirePlanApproval: true, planApprovalMode: "require-all" } as Settings,
+      );
+
+      expect(store.moveTask).toHaveBeenCalledWith(task.id, "todo");
+      expect(store.updateTask).not.toHaveBeenCalledWith(task.id, expect.objectContaining({ status: "awaiting-approval" }));
+      const persisted = readFileSync(join(rootDir, ".fusion", "tasks", task.id, "PROMPT.md"), "utf8");
+      expect(persisted).toContain("## Original Description");
+      expect(persisted).toContain("## Frontend UX Criteria");
+    });
+
     it("still re-asks approval for a CHANGED plan when the prior approval predates prompt hygiene", async () => {
       // The safety edge: legacy tolerance must not approve a plan the operator never saw.
       const legacyFingerprint = computePlanApprovalFingerprint(planText);

packages/engine/src/triage.ts

@@ -2951,6 +2951,14 @@ export class TriageProcessor {
        * already made their independent decisions, so it never weakens either of those gates
        * or auto-approve-all (which never reaches this branch at all).
        */
+      /*
+       * FNXC:PlanApproval 2026-07-15-21:05:
+       * FN-7569 / FN-8009 — compare the normalized as-approved fingerprint after deterministic
+       * Original Description or Frontend UX hygiene. approve-plan fingerprints the on-disk
+       * PROMPT.md, while recovery can receive pre-injection text; the shared hasher removes only
+       * those generated sections so an unchanged plan does not re-park. A genuinely changed plan
+       * still produces a different fingerprint and requires approval.
+       */
       const priorFingerprint = latestTransitionTask?.approvedPlanFingerprint ?? task.approvedPlanFingerprint;
       // FNXC:PlanApproval 2026-07-15-20:45: The shared hasher strips deterministic
       // Original Description / Frontend UX hygiene, so approve-plan's on-disk fingerprint and