feat(config): add workflow.use_worktrees toggle to disable worktree isolation

Adds `workflow.use_worktrees` config option (default: `true`) that
allows users to disable git worktree isolation for executor agents.

When set to `false`:
- Executor agents run without `isolation="worktree"`
- Plans execute sequentially on the main working tree
- No worktree merge ordering issues or orphaned worktrees
- Normal git hooks run (no --no-verify needed)

This provides an escape hatch for solo developers and users who
experience worktree merge conflicts, as worktree ordering issues
are inherently difficult when parallel agents modify overlapping
files.

Usage:
  /gsd:settings → set workflow.use_worktrees to false

Or directly:
  gsd-tools config-set workflow.use_worktrees false

Changes:
- config.cjs: add workflow.use_worktrees to valid keys
- planning-config.md: document the option
- execute-phase.md: read config, conditional worktree + sequential mode
- execute-plan.md: conditional worktree in Pattern A
- quick.md: conditional worktree for quick executor
- diagnose-issues.md: conditional worktree for debug agents
- 2 new tests (config set + workflow structural check)

Closes #1451

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

tests/execute-phase-wave.test.cjs

@@ -105,4 +105,20 @@ describe('execute-phase docs: user-facing wave flag', () => {
       'help.md should include wave-filter usage'
     );
   });
+
+  test('workflow supports use_worktrees config toggle', () => {
+    const content = fs.readFileSync(WORKFLOW_PATH, 'utf-8');
+    assert.ok(
+      content.includes('USE_WORKTREES'),
+      'workflow should reference USE_WORKTREES variable'
+    );
+    assert.ok(
+      content.includes('config-get workflow.use_worktrees'),
+      'workflow should read use_worktrees from config'
+    );
+    assert.ok(
+      content.includes('Sequential mode'),
+      'workflow should document sequential mode when worktrees disabled'
+    );
+  });
 });