fix(#751): recognise config-set prototype-pollution guard in CodeQL + test dynamic-key vectors (#752)

CodeQL alert #26 (js/prototype-pollution-utility) kept firing on setConfigValue
because its dataflow does not trace the #663 Set-based, pre-loop keys.some(...)
forbidden-key check as a sanitising barrier on the write site.

- src/config.cts: replace the Set + pre-loop check with inline literal
  comparisons (key === '__proto__' || 'prototype' || 'constructor') on the exact
  key used to index `current`, immediately before each write (intermediate keys
  in the descent loop, plus the final key). Same forbidden set, same error
  message and ERROR_REASON.CONFIG_PARSE_FAILED — behaviour unchanged from #663,
  but the barrier is now CodeQL-recognised.
- tests/config.test.cjs: add regression tests for schema-valid dynamic-prefix
  keys (agent_skills.__proto__, agent_skills.constructor, agent_skills.prototype,
  features.__proto__, review.models.constructor) that pass the isValidConfigKey
  schema gate and reach the guard. Each asserts the guard's own message fires
  (not the schema gate's "Unknown config key") and Object.prototype is not
  polluted. The prior #663 tests never reached the guard — their keys are
  rejected by the schema gate first — so the guard's real attack surface was
  untested.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

tests/config.test.cjs

@@ -1158,6 +1158,127 @@ describe('config-set prototype-pollution guard (#663)', () => {
   });
 });
 
+// ─── config-set prototype-pollution guard via dynamic-key prefixes (alert #26) ─
+
+describe('config-set prototype-pollution guard via dynamic-key prefixes (alert #26)', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = createTempProject();
+    // Initialise config so there is a config.json to write to.
+    runGsdTools('config-ensure-section', tmpDir);
+  });
+
+  afterEach(() => {
+    cleanup(tmpDir);
+  });
+
+  test('agent_skills.__proto__ is blocked by setConfigValue guard (not schema gate)', () => {
+    const result = runGsdTools('config-set agent_skills.__proto__ somevalue', tmpDir);
+
+    assert.strictEqual(result.success, false, `Expected failure but got: ${result.output}`);
+
+    // Must be the pollution guard, not the schema gate.
+    assert.ok(
+      result.error.includes('prototype pollution guard'),
+      `Expected "prototype pollution guard" in error, got: ${result.error}`,
+    );
+    // No schema-gate message.
+    assert.ok(
+      !result.error.includes('Unknown config key'),
+      `Should not hit schema gate, got: ${result.error}`,
+    );
+
+    // No prototype pollution occurred.
+    assert.strictEqual(({}).somevalue, undefined, 'agent_skills.__proto__: {}.somevalue should be undefined');
+    assert.strictEqual(Object.prototype.hasOwnProperty.call(Object.prototype, 'somevalue'), false,
+      'agent_skills.__proto__: Object.prototype should not gain "somevalue"');
+  });
+
+  test('agent_skills.constructor is blocked by setConfigValue guard (not schema gate)', () => {
+    const result = runGsdTools('config-set agent_skills.constructor somevalue', tmpDir);
+
+    assert.strictEqual(result.success, false, `Expected failure but got: ${result.output}`);
+
+    assert.ok(
+      result.error.includes('prototype pollution guard'),
+      `Expected "prototype pollution guard" in error, got: ${result.error}`,
+    );
+    assert.ok(
+      !result.error.includes('Unknown config key'),
+      `Should not hit schema gate, got: ${result.error}`,
+    );
+
+    assert.strictEqual(Object.prototype.hasOwnProperty.call(Object.prototype, 'somevalue'), false,
+      'agent_skills.constructor: Object.prototype should not gain "somevalue"');
+  });
+
+  test('agent_skills.prototype is blocked by setConfigValue guard (not schema gate)', () => {
+    const result = runGsdTools('config-set agent_skills.prototype somevalue', tmpDir);
+
+    assert.strictEqual(result.success, false, `Expected failure but got: ${result.output}`);
+
+    assert.ok(
+      result.error.includes('prototype pollution guard'),
+      `Expected "prototype pollution guard" in error, got: ${result.error}`,
+    );
+    assert.ok(
+      !result.error.includes('Unknown config key'),
+      `Should not hit schema gate, got: ${result.error}`,
+    );
+
+    assert.strictEqual(Object.prototype.hasOwnProperty.call(Object.prototype, 'somevalue'), false,
+      'agent_skills.prototype: Object.prototype should not gain "somevalue"');
+  });
+
+  test('features.__proto__ is blocked by setConfigValue guard (not schema gate)', () => {
+    const result = runGsdTools('config-set features.__proto__ somevalue', tmpDir);
+
+    assert.strictEqual(result.success, false, `Expected failure but got: ${result.output}`);
+
+    assert.ok(
+      result.error.includes('prototype pollution guard'),
+      `Expected "prototype pollution guard" in error, got: ${result.error}`,
+    );
+    assert.ok(
+      !result.error.includes('Unknown config key'),
+      `Should not hit schema gate, got: ${result.error}`,
+    );
+
+    assert.strictEqual(({}).somevalue, undefined, 'features.__proto__: {}.somevalue should be undefined');
+    assert.strictEqual(Object.prototype.hasOwnProperty.call(Object.prototype, 'somevalue'), false,
+      'features.__proto__: Object.prototype should not gain "somevalue"');
+  });
+
+  test('review.models.constructor is blocked by setConfigValue guard (not schema gate)', () => {
+    const result = runGsdTools('config-set review.models.constructor somevalue', tmpDir);
+
+    assert.strictEqual(result.success, false, `Expected failure but got: ${result.output}`);
+
+    assert.ok(
+      result.error.includes('prototype pollution guard'),
+      `Expected "prototype pollution guard" in error, got: ${result.error}`,
+    );
+    assert.ok(
+      !result.error.includes('Unknown config key'),
+      `Should not hit schema gate, got: ${result.error}`,
+    );
+
+    assert.strictEqual(Object.prototype.hasOwnProperty.call(Object.prototype, 'somevalue'), false,
+      'review.models.constructor: Object.prototype should not gain "somevalue"');
+  });
+
+  test('positive control: agent_skills.sonnet-coder with valid value succeeds', () => {
+    const result = runGsdTools('config-set agent_skills.sonnet-coder true', tmpDir);
+
+    assert.ok(result.success, `Legitimate agent_skills key rejected unexpectedly: ${result.error}`);
+
+    const config = readConfig(tmpDir);
+    assert.strictEqual(config.agent_skills['sonnet-coder'], true,
+      'agent_skills.sonnet-coder should be written to config.json');
+  });
+});
+
 // ─── plan_review.source_grounding + _authority (#22) ─────────────────────────
 
 describe('plan_review.source_grounding and plan_review.source_grounding_authority (#22)', () => {