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

.agents/rules/blog-system.md

@@ -54,3 +54,18 @@
    - Wikilinks `[[slug]]` or `[[slug|Label]]` must be transformed into standard web URLs (`/posts/slug/`).
    - Callout blocks (`> [!NOTE]`, `> [!WARNING]`, etc.) must render as terminal-style alert callouts.
    - Images must be processed into `src/assets/images/posts/` and linked via standard `<img>` or Markdown image syntax.
+
+---
+
+## 5. Test-Driven Development (TDD) & Adversarial Verification
+1. **Red-Green-Refactor Lifecycle:**
+   - Write tests in `test/*.test.js` using Node's native test runner (`node:test`, `node:assert/strict`).
+   - Test inputs before implementing filters, data loaders, or template macros.
+2. **Adversarial Test Coverage Required:**
+   - **Data & Configuration:** Test that missing required environment variables (e.g. `SITE_DOMAIN`) throw fatal errors.
+   - **Filters:** Test edge cases: empty strings, undefined values, single-word inputs, massive word counts, non-date objects.
+   - **Content Schemas:** Validate all Markdown files in `src/posts/` against frontmatter schemas.
+   - **Design Spec Compliance:** Automate regex/AST assertions on `src/assets/css/` ensuring zero violations of the "Do Not Use" list (`backdrop-filter`, `box-shadow`, `text-shadow`, gradients, floaty transitions).
+   - **Link & Anchor Integrity:** Verify that every compiled page has zero broken internal links and that all Table of Contents anchor links resolve to valid heading IDs.
+3. **Execution Invariant:**
+   - Always run `npm test` before committing. All tests must pass cleanly with 0 failures.