feat: add three guardrails against out-of-scope agent deletions

1. Scoping rules in agent prompts: executor, triage template, and reviewer
   now explicitly forbid deleting/gutting modules, settings, interfaces,
   exports, or test files outside the task's declared File Scope. Reviewer
   will REVISE if out-of-scope removals are detected.

2. Pre-merge diffstat scope check: merger.ts validates the git diffstat
   against the task's PROMPT.md File Scope before merging. Large deletions
   outside scope are logged as warnings on the task (soft guardrail).

3. Changeset requirement for feature removal: triage template now requires
   a .changeset/ entry when removing existing functionality. Executor and
   reviewer enforce this requirement.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/engine/src/merger.test.ts

@@ -39,6 +39,9 @@ import {
   resolveTrivialWhitespace,
   LOCKFILE_PATTERNS,
   GENERATED_PATTERNS,
+  parseDiffStat,
+  extractFileScope,
+  validateDiffScope,
   type ConflictCategory,
 } from "./merger.js";
 import { createKbAgent } from "./pi.js";
@@ -1960,3 +1963,172 @@ describe("aiMergeTask — build verification", () => {
     expect(store.moveTask).toHaveBeenCalledWith("FN-050", "done");
   });
 });
+
+// ── Pre-merge diffstat scope validation tests ────────────────────────
+
+describe("parseDiffStat", () => {
+  it("parses standard diffstat output", () => {
+    const stat = [
+      " packages/core/src/types.ts         | 9 ++--",
+      " packages/engine/src/notifier.ts     | 46 +-----",
+      " 2 files changed, 10 insertions(+), 45 deletions(-)",
+    ].join("\n");
+
+    const entries = parseDiffStat(stat);
+    expect(entries).toHaveLength(2);
+    expect(entries[0].file).toBe("packages/core/src/types.ts");
+    // Rounding may shift total by ±1, so check approximate range
+    expect(entries[0].insertions + entries[0].deletions).toBeGreaterThanOrEqual(9);
+    expect(entries[0].insertions + entries[0].deletions).toBeLessThanOrEqual(10);
+    expect(entries[1].file).toBe("packages/engine/src/notifier.ts");
+    expect(entries[1].deletions).toBeGreaterThan(entries[1].insertions);
+  });
+
+  it("handles pure-deletion lines", () => {
+    const stat = " packages/engine/src/usage.ts | 527 ---";
+    const entries = parseDiffStat(stat);
+    expect(entries).toHaveLength(1);
+    expect(entries[0].insertions).toBe(0);
+    expect(entries[0].deletions).toBe(527);
+  });
+
+  it("handles pure-insertion lines", () => {
+    const stat = " packages/engine/src/new.ts | 100 +++";
+    const entries = parseDiffStat(stat);
+    expect(entries).toHaveLength(1);
+    expect(entries[0].insertions).toBe(100);
+    expect(entries[0].deletions).toBe(0);
+  });
+
+  it("returns empty for unreadable stat", () => {
+    expect(parseDiffStat("(unable to read diff)")).toEqual([]);
+    expect(parseDiffStat("")).toEqual([]);
+  });
+
+  it("skips summary line", () => {
+    const stat = " 1 file changed, 5 insertions(+)";
+    expect(parseDiffStat(stat)).toEqual([]);
+  });
+});
+
+describe("extractFileScope", () => {
+  it("extracts file patterns from PROMPT.md", () => {
+    const prompt = [
+      "# Task: FN-100 - Add feature",
+      "",
+      "## File Scope",
+      "",
+      "- `packages/core/src/types.ts`",
+      "- `packages/engine/src/notifier.ts`",
+      "- `packages/dashboard/app/components/*`",
+      "",
+      "## Steps",
+      "",
+      "### Step 1: Do things",
+    ].join("\n");
+
+    const scope = extractFileScope(prompt);
+    expect(scope).toEqual([
+      "packages/core/src/types.ts",
+      "packages/engine/src/notifier.ts",
+      "packages/dashboard/app/components/*",
+    ]);
+  });
+
+  it("handles patterns with artifact annotations", () => {
+    const prompt = [
+      "## File Scope",
+      "",
+      "- `src/foo.ts` (new)",
+      "- `src/bar.ts` (modified)",
+      "",
+      "## Steps",
+    ].join("\n");
+
+    const scope = extractFileScope(prompt);
+    expect(scope).toEqual(["src/foo.ts", "src/bar.ts"]);
+  });
+
+  it("returns empty for missing File Scope section", () => {
+    const prompt = "# Task\n\n## Steps\n### Step 1\n";
+    expect(extractFileScope(prompt)).toEqual([]);
+  });
+});
+
+describe("validateDiffScope", () => {
+  it("returns warnings for large deletions outside scope", async () => {
+    const store = {
+      getTask: vi.fn().mockResolvedValue({
+        prompt: [
+          "## File Scope",
+          "",
+          "- `packages/dashboard/app/components/Header.tsx`",
+          "",
+          "## Steps",
+        ].join("\n"),
+      }),
+      logEntry: vi.fn(),
+    } as unknown as TaskStore;
+
+    const diffStat = [
+      " packages/dashboard/app/components/Header.tsx | 20 ++--",
+      " packages/engine/src/usage.ts                 | 527 ---",
+      " packages/engine/src/usage.test.ts            | 524 ---",
+      " 3 files changed, 5 insertions(+), 1066 deletions(-)",
+    ].join("\n");
+
+    const result = await validateDiffScope(store, "FN-100", diffStat);
+    expect(result.outOfScopeFiles).toContain("packages/engine/src/usage.ts");
+    expect(result.outOfScopeFiles).toContain("packages/engine/src/usage.test.ts");
+    expect(result.largeOutOfScopeDeletions).toHaveLength(2);
+    expect(result.warnings.length).toBeGreaterThan(0);
+    expect(result.warnings[0]).toContain("SCOPE WARNING");
+  });
+
+  it("allows changeset files outside scope", async () => {
+    const store = {
+      getTask: vi.fn().mockResolvedValue({
+        prompt: "## File Scope\n\n- `src/foo.ts`\n\n## Steps",
+      }),
+    } as unknown as TaskStore;
+
+    const diffStat = [
+      " src/foo.ts                          | 10 +++",
+      " .changeset/my-change.md             | 5 +++",
+      " 2 files changed, 15 insertions(+)",
+    ].join("\n");
+
+    const result = await validateDiffScope(store, "FN-100", diffStat);
+    expect(result.outOfScopeFiles).not.toContain(".changeset/my-change.md");
+    expect(result.warnings).toHaveLength(0);
+  });
+
+  it("returns empty result when no scope is declared", async () => {
+    const store = {
+      getTask: vi.fn().mockResolvedValue({
+        prompt: "# Task\n\n## Steps\n",
+      }),
+    } as unknown as TaskStore;
+
+    const result = await validateDiffScope(store, "FN-100", " foo.ts | 500 ---");
+    expect(result.warnings).toHaveLength(0);
+  });
+
+  it("does not warn for in-scope changes", async () => {
+    const store = {
+      getTask: vi.fn().mockResolvedValue({
+        prompt: "## File Scope\n\n- `packages/engine/src/*`\n\n## Steps",
+      }),
+    } as unknown as TaskStore;
+
+    const diffStat = [
+      " packages/engine/src/executor.ts | 50 +++---",
+      " packages/engine/src/triage.ts   | 30 +++---",
+      " 2 files changed, 40 insertions(+), 40 deletions(-)",
+    ].join("\n");
+
+    const result = await validateDiffScope(store, "FN-100", diffStat);
+    expect(result.outOfScopeFiles).toHaveLength(0);
+    expect(result.warnings).toHaveLength(0);
+  });
+});