fix(federation): harness round-2 — email validation + host-side URL rewrite

- Bug-1: replace whitespace in admin email local-part (was breaking @IsEmail)
- Bug-2: rewrite enrollment URL to use host-accessible base in seed.ts (in-cluster URL not resolvable from host)
- Bug-3: correct README Known Limitations section
- eslint.config.mjs: add tools/federation-harness/*.ts to allowDefaultProject so pre-commit hook can lint harness scripts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

eslint.config.mjs

@@ -30,6 +30,7 @@ export default tseslint.config(
             'apps/gateway/vitest.config.ts',
             'packages/storage/vitest.config.ts',
             'packages/mosaic/__tests__/*.ts',
+            'tools/federation-harness/*.ts',
           ],
         },
       },