FN-5659: add deterministic goal citation proof test

Add a deterministic mock-runtime proof path that verifies executor output cites injected active goal IDs.

- add a new engine test for goal-citation proof scaffolding and emitted goal ID extraction
- validate positive behavior when ## Active Goals is present in session system prompt
- add a negative control confirming no citation is emitted without injected goal context
- assert the proof runs on mock/scripted runtime for deterministic, model-free coverage

Files changed:
 .../src/__tests__/goal-citation-proof.test.ts      | 142 +++++++++++++++++++++
 1 file changed, 142 insertions(+)

Fusion-Task-Id: FN-5659

Fusion-Task-Lineage: 35f7c1ec-8de3-4813-8f9e-fc5f72096d91

packages/engine/src/__tests__/goal-citation-proof.test.ts

@@ -0,0 +1,142 @@
+/**
+ * Goal-citation proof (FN-5659)
+ *
+ * Deterministic Slice 2 proof path: verifies an executor-lane mock session can
+ * observe injected `## Active Goals` context and cite a goal ID in output.
+ * It runs entirely on `MockAgentRuntime` (`mock/scripted`) under `pnpm test`,
+ * with no real model dependency. Citation evidence is the captured `onText`
+ * emission asserted in the positive case, with a negative no-goal control.
+ *
+ * This test lives with mock-provider coverage so operators can discover it via:
+ * `grep -R "Active Goals" packages/engine/src/__tests__/`
+ */
+import { mkdtemp } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import type { Goal } from "@fusion/core";
+import { buildGoalContextSection } from "../goal-context-injector.js";
+import { MockAgentRuntime, resetMockScripts, setMockScript } from "../providers/mock-provider.js";
+
+const GOAL_ID_PATTERN = /^- (G-[A-Za-z0-9-]+):/gm;
+
+const fixtureGoals: Goal[] = [
+  {
+    id: "G-0001",
+    title: "Increase plugin ecosystem adoption",
+    description: "",
+    status: "active",
+    createdAt: "2026-05-01T00:00:00.000Z",
+    updatedAt: "2026-05-01T00:00:00.000Z",
+  },
+  {
+    id: "G-0002",
+    title: "Improve task-board execution reliability",
+    description: "",
+    status: "active",
+    createdAt: "2026-05-02T00:00:00.000Z",
+    updatedAt: "2026-05-02T00:00:00.000Z",
+  },
+  {
+    id: "G-0003",
+    title: "Harden deterministic regression coverage",
+    description: "",
+    status: "active",
+    createdAt: "2026-05-03T00:00:00.000Z",
+    updatedAt: "2026-05-03T00:00:00.000Z",
+  },
+];
+
+describe("goal-citation proof scaffolding", () => {
+  beforeEach(() => {
+    resetMockScripts();
+  });
+
+  afterEach(() => {
+    resetMockScripts();
+  });
+
+  it("builds a deterministic Active Goals section with emitted IDs", () => {
+    const result = buildGoalContextSection({ activeGoals: fixtureGoals });
+
+    expect(result.text).toContain("## Active Goals");
+    for (const goal of fixtureGoals) {
+      expect(result.text).toContain(goal.id);
+    }
+    expect(result.emittedGoalIds).toEqual(["G-0001", "G-0002", "G-0003"]);
+  });
+
+  it("emits a goal citation only when the Active Goals block exists", async () => {
+    const runtime = new MockAgentRuntime();
+    const cwd = await mkdtemp(join(tmpdir(), "fn-goal-citation-proof-"));
+    const result = buildGoalContextSection({ activeGoals: fixtureGoals });
+    const onText = vi.fn();
+    const proofTaskId = "FN-CITE-PROOF";
+
+    setMockScript(
+      { sessionPurpose: "executor", taskId: proofTaskId },
+      {
+        run: async (ctx) => {
+          // Goal anchors live in options.systemPrompt (session-level), not per-turn ctx.prompt.
+          const matches = Array.from((ctx.options.systemPrompt ?? "").matchAll(GOAL_ID_PATTERN));
+          const ids = matches.map((match) => match[1]);
+          const firstId = ids[0];
+          if (firstId) {
+            ctx.options.onText?.(`Citing goal ${firstId} when prioritizing plugin work.`);
+            return;
+          }
+          ctx.options.onText?.("NO_GOAL_CITATION");
+        },
+      },
+    );
+
+    const systemPrompt = `Task context: prioritize plugin impact.\n\n${result.text}`;
+    const { session } = await runtime.createSession({
+      cwd,
+      systemPrompt,
+      runtimeContext: { sessionPurpose: "executor" },
+      customTools: [],
+      onText,
+      taskId: proofTaskId,
+      taskTitle: "Goal citation proof",
+    });
+
+    await runtime.promptWithFallback(session, "Pick the highest-leverage next step.");
+
+    const emissions = onText.mock.calls.map(([text]) => String(text));
+    expect(emissions.some((text) => text.includes(result.emittedGoalIds[0]))).toBe(true);
+    expect(emissions.some((text) => /Citing goal G-\S+/.test(text))).toBe(true);
+    expect(runtime.describeModel(session)).toBe("mock/scripted");
+
+    const noGoalTaskId = "FN-NO-GOALS";
+    const onTextNoGoals = vi.fn();
+    setMockScript(
+      { sessionPurpose: "executor", taskId: noGoalTaskId },
+      {
+        run: async (ctx) => {
+          const matches = Array.from((ctx.options.systemPrompt ?? "").matchAll(GOAL_ID_PATTERN));
+          const ids = matches.map((match) => match[1]);
+          const firstId = ids[0];
+          if (firstId) {
+            ctx.options.onText?.(`Citing goal ${firstId} when prioritizing plugin work.`);
+            return;
+          }
+          ctx.options.onText?.("NO_GOAL_CITATION");
+        },
+      },
+    );
+
+    const { session: noGoalSession } = await runtime.createSession({
+      cwd,
+      systemPrompt: "Task context: no injected goal context.",
+      runtimeContext: { sessionPurpose: "executor" },
+      customTools: [],
+      onText: onTextNoGoals,
+      taskId: noGoalTaskId,
+      taskTitle: "Goal citation negative proof",
+    });
+
+    await runtime.promptWithFallback(noGoalSession, "Pick the highest-leverage next step.");
+    expect(onTextNoGoals).toHaveBeenCalledWith("NO_GOAL_CITATION");
+  });
+});