feat(FN-4604): complete Step 1 — add milestone acceptance derivation helper

Fusion-Task-Id: FN-4604
Fusion-Task-Lineage: 71752cd0-eadb-4434-a2a3-d9fff562ea63

packages/core/src/__tests__/mission-store.test.ts

@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, afterEach } from "vitest";
-import { MissionStore } from "../mission-store.js";
+import { MissionStore, deriveMilestoneAcceptanceCriteriaFromFeatures } from "../mission-store.js";
 import { Database } from "../db.js";
+import type { MissionFeature } from "../mission-types.js";
 import { mkdtempSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
@@ -916,6 +917,77 @@ describe("MissionStore", () => {
     });
   });
 
+  describe("milestone acceptance criteria derivation", () => {
+    const makeFeature = (overrides: Partial<MissionFeature>): MissionFeature => ({
+      id: "F-1",
+      sliceId: "SL-1",
+      title: "Feature",
+      status: "defined",
+      createdAt: new Date().toISOString(),
+      updatedAt: new Date().toISOString(),
+      ...overrides,
+    });
+
+    it("derives milestone acceptance from feature acceptance criteria", () => {
+      const derived = deriveMilestoneAcceptanceCriteriaFromFeatures([
+        makeFeature({ title: "Login", acceptanceCriteria: "  Auth succeeds  " }),
+      ]);
+
+      expect(derived).toBe("- Login: Auth succeeds");
+    });
+
+    it("falls back to feature description when acceptance criteria is blank", () => {
+      const derived = deriveMilestoneAcceptanceCriteriaFromFeatures([
+        makeFeature({ title: "Login", acceptanceCriteria: "   ", description: "  Works across browsers " }),
+      ]);
+
+      expect(derived).toBe("- Login: Works across browsers");
+    });
+
+    it("skips features without acceptance text and returns undefined when none contribute", () => {
+      const derived = deriveMilestoneAcceptanceCriteriaFromFeatures([
+        makeFeature({ title: "Login", acceptanceCriteria: "", description: "   " }),
+      ]);
+
+      expect(derived).toBeUndefined();
+    });
+
+    it("does not overwrite explicit milestone acceptance criteria", () => {
+      const mission = store.createMission({ title: "Mission" });
+      const milestone = store.addMilestone(mission.id, {
+        title: "Milestone",
+        acceptanceCriteria: "Explicit criteria",
+      });
+      const slice = store.addSlice(milestone.id, { title: "Slice" });
+      store.addFeature(slice.id, {
+        title: "Feature",
+        acceptanceCriteria: "Feature criteria",
+      });
+
+      const updated = store.applyDerivedMilestoneAcceptanceCriteria(milestone.id);
+      expect(updated.acceptanceCriteria).toBe("Explicit criteria");
+    });
+
+    it("preserves explicit milestone criteria when re-applied after feature changes", () => {
+      const mission = store.createMission({ title: "Mission" });
+      const milestone = store.addMilestone(mission.id, { title: "Milestone" });
+      const slice = store.addSlice(milestone.id, { title: "Slice" });
+      const feature = store.addFeature(slice.id, {
+        title: "Feature",
+        acceptanceCriteria: "Initial acceptance",
+      });
+
+      const firstDerived = store.applyDerivedMilestoneAcceptanceCriteria(milestone.id);
+      expect(firstDerived.acceptanceCriteria).toBe("- Feature: Initial acceptance");
+
+      store.updateMilestone(milestone.id, { acceptanceCriteria: "Manual lock" });
+      store.updateFeature(feature.id, { acceptanceCriteria: "Changed acceptance" });
+
+      const preserved = store.applyDerivedMilestoneAcceptanceCriteria(milestone.id);
+      expect(preserved.acceptanceCriteria).toBe("Manual lock");
+    });
+  });
+
   // ── Slice CRUD Tests ──────────────────────────────────────────────────
 
   describe("Slice CRUD", () => {

packages/core/src/mission-store.ts

@@ -62,6 +62,32 @@ import {
  */
 const DEFAULT_IMPLEMENTATION_RETRY_BUDGET = 3;
 
+export function deriveMilestoneAcceptanceCriteriaFromFeatures(features: MissionFeature[]): string | undefined {
+  const lines = features
+    .map((feature) => {
+      const acceptance = feature.acceptanceCriteria?.trim();
+      const description = feature.description?.trim();
+      const text = acceptance && acceptance.length > 0
+        ? acceptance
+        : description && description.length > 0
+          ? description
+          : undefined;
+
+      if (!text) {
+        return undefined;
+      }
+
+      return `- ${feature.title}: ${text}`;
+    })
+    .filter((line): line is string => Boolean(line));
+
+  if (lines.length === 0) {
+    return undefined;
+  }
+
+  return lines.join("\n");
+}
+
 // ── Mission Summary Type ─────────────────────────────────────────────
 
 /** Status summary for a mission, computed from its hierarchy. */
@@ -1341,6 +1367,26 @@ export class MissionStore extends EventEmitter<MissionStoreEvents> {
     return this.updateMilestone(id, { interviewState: state });
   }
 
+  applyDerivedMilestoneAcceptanceCriteria(milestoneId: string): Milestone {
+    const milestone = this.getMilestone(milestoneId);
+    if (!milestone) {
+      throw new Error(`Milestone ${milestoneId} not found`);
+    }
+
+    if (milestone.acceptanceCriteria?.trim()) {
+      return milestone;
+    }
+
+    const features = this.listSlices(milestoneId).flatMap((slice) => this.listFeatures(slice.id));
+    const derivedAcceptanceCriteria = deriveMilestoneAcceptanceCriteriaFromFeatures(features);
+
+    if (!derivedAcceptanceCriteria) {
+      return milestone;
+    }
+
+    return this.updateMilestone(milestoneId, { acceptanceCriteria: derivedAcceptanceCriteria });
+  }
+
   // ── Slice Operations ───────────────────────────────────────────────
 
   /**
@@ -1664,6 +1710,7 @@ export class MissionStore extends EventEmitter<MissionStoreEvents> {
     // update the parent milestone and mission statuses. Calling recomputeSliceStatus
     // here ensures the full chain is updated atomically when a feature is added.
     this.recomputeSliceStatus(sliceId);
+    this.applyDerivedMilestoneAcceptanceCriteria(slice.milestoneId);
 
     return feature;
   }