fix(execute): use sequential dispatch instead of timing-based stagger (#1541)

Address review feedback: LLMs cannot enforce timing between tool calls.
Replace "stagger by ~2 seconds" with concrete, enforceable pattern:
dispatch each Task() one at a time with run_in_background: true. The
round-trip latency of each tool call provides natural spacing for
worktree creation, while agents still run in parallel once created.
Explicitly warn against sending multiple Task() calls in a single
message (which causes simultaneous git worktree add).

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

tests/worktree-stagger.test.cjs

@@ -0,0 +1,49 @@
+/**
+ * GSD Worktree Sequential Dispatch Tests
+ *
+ * Validates that execute-phase workflow includes sequential dispatch
+ * instructions to prevent git config.lock contention when multiple
+ * agents create worktrees in parallel within the same wave.
+ *
+ * See: https://github.com/gsd-build/get-shit-done/issues/1511
+ */
+
+const { test, describe } = require('node:test');
+const assert = require('node:assert');
+const fs = require('fs');
+const path = require('path');
+
+const WORKFLOWS_DIR = path.join(__dirname, '..', 'get-shit-done', 'workflows');
+
+describe('worktree sequential dispatch', () => {
+  const executePhasePath = path.join(WORKFLOWS_DIR, 'execute-phase.md');
+  let content;
+
+  test('execute-phase.md exists', () => {
+    assert.ok(fs.existsSync(executePhasePath), 'execute-phase.md should exist');
+  });
+
+  test('execute-phase explains git config.lock contention', () => {
+    content = fs.readFileSync(executePhasePath, 'utf-8');
+    assert.ok(
+      content.includes('config.lock'),
+      'execute-phase.md should explain the git config.lock race condition'
+    );
+  });
+
+  test('execute-phase requires sequential dispatch with run_in_background', () => {
+    content = content || fs.readFileSync(executePhasePath, 'utf-8');
+    assert.ok(
+      content.includes('run_in_background'),
+      'execute-phase.md should instruct one-at-a-time dispatch with run_in_background'
+    );
+  });
+
+  test('execute-phase warns against multiple Task calls in single message', () => {
+    content = content || fs.readFileSync(executePhasePath, 'utf-8');
+    assert.ok(
+      content.includes('WRONG') && content.includes('single message'),
+      'execute-phase.md should warn against sending multiple Task() calls simultaneously'
+    );
+  });
+});