test(02-02): add frontmatter merge and validate CLI integration tests

- frontmatter merge: multi-field merge, conflict overwrite, missing file, invalid JSON
- frontmatter validate: all 3 schemas (plan, summary, verification), missing fields
  detection with count, unknown schema error, missing file error

tests/frontmatter-cli.test.cjs

@@ -134,3 +134,138 @@ describe('frontmatter set', () => {
     assert.ok(content.includes('Some paragraph with special chars: $, %, &.'), 'body content should be preserved');
   });
 });
+
+// ─── frontmatter merge ──────────────────────────────────────────────────────
+
+describe('frontmatter merge', () => {
+  test('merges multiple fields into frontmatter', () => {
+    const file = writeTempFile('---\nphase: 01\n---\nbody');
+    const result = runGsdTools(`frontmatter merge ${file} --data '{"plan":"02","type":"tdd"}'`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const content = fs.readFileSync(file, 'utf-8');
+    const { extractFrontmatter } = require('../get-shit-done/bin/lib/frontmatter.cjs');
+    const fm = extractFrontmatter(content);
+    assert.strictEqual(fm.phase, '01', 'original field should be preserved');
+    assert.strictEqual(fm.plan, '02', 'merged field should be present');
+    assert.strictEqual(fm.type, 'tdd', 'merged field should be present');
+  });
+
+  test('overwrites existing fields on conflict', () => {
+    const file = writeTempFile('---\nphase: 01\ntype: execute\n---\nbody');
+    const result = runGsdTools(`frontmatter merge ${file} --data '{"phase":"02"}'`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const content = fs.readFileSync(file, 'utf-8');
+    const { extractFrontmatter } = require('../get-shit-done/bin/lib/frontmatter.cjs');
+    const fm = extractFrontmatter(content);
+    assert.strictEqual(fm.phase, '02', 'conflicting field should be overwritten');
+    assert.strictEqual(fm.type, 'execute', 'non-conflicting field should be preserved');
+  });
+
+  test('returns error for missing file', () => {
+    const result = runGsdTools(`frontmatter merge /nonexistent/file.md --data '{"phase":"01"}'`);
+    assert.ok(result.success, 'Command should exit 0 with error JSON');
+    const parsed = JSON.parse(result.output);
+    assert.ok(parsed.error, 'Should have error field');
+  });
+
+  test('returns error for invalid JSON data', () => {
+    const file = writeTempFile('---\nphase: 01\n---\nbody');
+    const result = runGsdTools(`frontmatter merge ${file} --data 'not json'`);
+    // cmdFrontmatterMerge calls error() which exits with code 1
+    assert.ok(!result.success, 'Command should fail with non-zero exit code');
+    assert.ok(result.error.includes('Invalid JSON'), 'Error should mention invalid JSON');
+  });
+});
+
+// ─── frontmatter validate ───────────────────────────────────────────────────
+
+describe('frontmatter validate', () => {
+  test('reports valid for complete plan frontmatter', () => {
+    const content = `---
+phase: 01
+plan: 01
+type: execute
+wave: 1
+depends_on: []
+files_modified: [src/auth.ts]
+autonomous: true
+must_haves:
+  truths:
+    - "All tests pass"
+---
+body`;
+    const file = writeTempFile(content);
+    const result = runGsdTools(`frontmatter validate ${file} --schema plan`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+    const parsed = JSON.parse(result.output);
+    assert.strictEqual(parsed.valid, true, 'Should be valid');
+    assert.deepStrictEqual(parsed.missing, [], 'No fields should be missing');
+    assert.strictEqual(parsed.schema, 'plan');
+  });
+
+  test('reports invalid with missing fields', () => {
+    const file = writeTempFile('---\nphase: 01\n---\nbody');
+    const result = runGsdTools(`frontmatter validate ${file} --schema plan`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+    const parsed = JSON.parse(result.output);
+    assert.strictEqual(parsed.valid, false, 'Should be invalid');
+    assert.ok(parsed.missing.length > 0, 'Should have missing fields');
+    // plan schema requires: phase, plan, type, wave, depends_on, files_modified, autonomous, must_haves
+    // phase is present, so 7 should be missing
+    assert.strictEqual(parsed.missing.length, 7, 'Should have 7 missing required fields');
+    assert.ok(parsed.missing.includes('plan'), 'plan should be in missing');
+    assert.ok(parsed.missing.includes('type'), 'type should be in missing');
+    assert.ok(parsed.missing.includes('must_haves'), 'must_haves should be in missing');
+  });
+
+  test('validates against summary schema', () => {
+    const content = `---
+phase: 01
+plan: 01
+subsystem: testing
+tags: [unit-tests, yaml]
+duration: 5min
+completed: 2026-02-25
+---
+body`;
+    const file = writeTempFile(content);
+    const result = runGsdTools(`frontmatter validate ${file} --schema summary`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+    const parsed = JSON.parse(result.output);
+    assert.strictEqual(parsed.valid, true, 'Should be valid for summary schema');
+    assert.strictEqual(parsed.schema, 'summary');
+  });
+
+  test('validates against verification schema', () => {
+    const content = `---
+phase: 01
+verified: 2026-02-25
+status: passed
+score: 5/5
+---
+body`;
+    const file = writeTempFile(content);
+    const result = runGsdTools(`frontmatter validate ${file} --schema verification`);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+    const parsed = JSON.parse(result.output);
+    assert.strictEqual(parsed.valid, true, 'Should be valid for verification schema');
+    assert.strictEqual(parsed.schema, 'verification');
+  });
+
+  test('returns error for unknown schema', () => {
+    const file = writeTempFile('---\nphase: 01\n---\n');
+    const result = runGsdTools(`frontmatter validate ${file} --schema unknown`);
+    // cmdFrontmatterValidate calls error() which exits with code 1
+    assert.ok(!result.success, 'Command should fail with non-zero exit code');
+    assert.ok(result.error.includes('Unknown schema'), 'Error should mention unknown schema');
+  });
+
+  test('returns error for missing file', () => {
+    const result = runGsdTools('frontmatter validate /nonexistent/file.md --schema plan');
+    assert.ok(result.success, 'Command should exit 0 with error JSON');
+    const parsed = JSON.parse(result.output);
+    assert.ok(parsed.error, 'Should have error field');
+  });
+});