test(config): add config-get/set roundtrip and cross-workflow structural tests for use_worktrees

Add comprehensive test coverage for the workflow.use_worktrees config toggle:
- config-get returns false after setting to false (roundtrip verification)
- config-get errors with "Key not found" when not set (validates workflow
  fallback behavior where `|| echo "true"` provides the default)
- config-get returns true after setting to true
- Toggle back and forth works correctly
- Structural tests verify USE_WORKTREES is wired into quick.md,
  diagnose-issues.md, execute-plan.md, planning-config.md, and config.cjs

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

tests/config.test.cjs

@@ -775,3 +775,58 @@ describe('config-set workflow.skip_discuss', () => {
     assert.strictEqual(output, true);
   });
 });
+
+// ─── config-set/config-get workflow.use_worktrees ────────────────────────────
+
+describe('config-set/config-get workflow.use_worktrees', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = createTempProject();
+    runGsdTools('config-ensure-section', tmpDir, { HOME: tmpDir, USERPROFILE: tmpDir });
+  });
+
+  afterEach(() => {
+    cleanup(tmpDir);
+  });
+
+  test('config-get workflow.use_worktrees returns false after setting to false', () => {
+    runGsdTools('config-set workflow.use_worktrees false', tmpDir);
+    const result = runGsdTools('config-get workflow.use_worktrees', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output, false);
+  });
+
+  test('config-get workflow.use_worktrees errors when not set (default config)', () => {
+    // config-ensure-section does NOT include use_worktrees in hardcoded defaults,
+    // so config-get should error with "Key not found". This is the expected behavior
+    // that workflows rely on: the shell fallback `|| echo "true"` provides the default.
+    const result = runGsdTools('config-get workflow.use_worktrees', tmpDir);
+    assert.strictEqual(result.success, false);
+    assert.ok(
+      result.error.includes('Key not found'),
+      `Expected "Key not found" in error: ${result.error}`
+    );
+  });
+
+  test('config-get workflow.use_worktrees returns true after setting to true', () => {
+    runGsdTools('config-set workflow.use_worktrees true', tmpDir);
+    const result = runGsdTools('config-get workflow.use_worktrees', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output, true);
+  });
+
+  test('use_worktrees can be toggled back and forth', () => {
+    runGsdTools('config-set workflow.use_worktrees false', tmpDir);
+    runGsdTools('config-set workflow.use_worktrees true', tmpDir);
+    const result = runGsdTools('config-get workflow.use_worktrees', tmpDir);
+    assert.ok(result.success, `Command failed: ${result.error}`);
+
+    const output = JSON.parse(result.output);
+    assert.strictEqual(output, true);
+  });
+});