fix: align new-project map gate and guard onboarding summary overwrite

init new-project now uses the same seven-file codebase map completeness
check as init onboard, so partial .planning/codebase/ directories no longer
skip the brownfield mapping offer after onboarding warns about an incomplete
map.

The onboard workflow now branches on onboarding_summary_exists and asks for
confirmation before regenerating SUMMARY.md on repeat runs.

tests/init.test.cjs

@@ -1599,6 +1599,9 @@ describe('cmdInitNewProject', () => {
   test('brownfield with codebase map does not need map', () => {
     fs.writeFileSync(path.join(tmpDir, 'package.json'), '{"name":"test"}');
     fs.mkdirSync(path.join(tmpDir, '.planning', 'codebase'), { recursive: true });
+    for (const name of ['STACK', 'ARCHITECTURE', 'STRUCTURE', 'CONVENTIONS', 'TESTING', 'INTEGRATIONS', 'CONCERNS']) {
+      fs.writeFileSync(path.join(tmpDir, '.planning', 'codebase', `${name}.md`), `# ${name}\n`);
+    }
 
     const result = runGsdTools('init new-project', tmpDir);
     assert.ok(result.success, `Command failed: ${result.error}`);