fix(execute-phase): add Copilot sequential fallback and spot-check completion detection (#1128)

- Default Copilot runtime to sequential inline execution instead of
  unreliable subagent spawning
- Add spot-check fallback in step 3 (SUMMARY.md exists + git commits
  found) so orchestrator never blocks indefinitely on missing completion
  signals
- Add runtime detection guidance in init step to force sequential mode
  under Copilot
- Add regression test verifying execute-phase contains Copilot fallback
  and spot-check documentation

tests/agent-frontmatter.test.cjs

@@ -151,6 +151,20 @@ describe('SPAWN: spawn type consistency', () => {
       'diagnose-issues should spawn gsd-debugger, not general-purpose'
     );
   });
+
+  test('execute-phase has Copilot sequential fallback in runtime_compatibility', () => {
+    const content = fs.readFileSync(
+      path.join(WORKFLOWS_DIR, 'execute-phase.md'), 'utf-8'
+    );
+    assert.ok(
+      content.includes('sequential inline execution'),
+      'execute-phase must document sequential inline execution as Copilot fallback'
+    );
+    assert.ok(
+      content.includes('spot-check'),
+      'execute-phase must have spot-check fallback for completion detection'
+    );
+  });
 });
 
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