feat: adopt strict TDD framework and add adversarial test suite

- Update AGENTS.md and .agents/rules/blog-system.md with strict TDD mandate
- Extract Eleventy custom filters into modular src/_filters/index.js for unit testing
- Add npm test script using Node.js native test runner (node:test)
- Create test/site-config.test.js for environment validation and fatal error assertions
- Create test/filters.test.js for adversarial filter edge cases (dates, limits, reading time, headings)
- Create test/frontmatter-schema.test.js enforcing post metadata contracts
- Create test/design-rules.test.js enforcing strict anti-patterns (no glassmorphism, no shadows, max 4px radius)
- Create test/build-integrity.test.js checking static output, zero 404 links, and TOC anchor targets

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "dev": "eleventy --serve --watch",
     "build": "eleventy",
+    "test": "node --test 'test/**/*.test.js'",
     "clean": "rm -rf _site",
     "start": "npm run dev"
   },