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

test/frontmatter-schema.test.js

@@ -0,0 +1,71 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import path from "node:path";
+
+const postsDir = path.resolve("src/posts");
+
+test("Content Contract: All posts in src/posts/ satisfy frontmatter schema", () => {
+  assert.ok(fs.existsSync(postsDir), "src/posts directory must exist");
+  const postFiles = fs.readdirSync(postsDir).filter((f) => f.endsWith(".md"));
+
+  assert.ok(postFiles.length > 0, "There must be at least one published post");
+
+  for (const file of postFiles) {
+    const filePath = path.join(postsDir, file);
+    const content = fs.readFileSync(filePath, "utf-8");
+
+    // 1. Filename Convention: YYYY-MM-DD-slug.md
+    assert.match(
+      file,
+      /^\d{4}-\d{2}-\d{2}-[a-z0-9-]+\.md$/,
+      `Post filename '${file}' must follow 'YYYY-MM-DD-slug.md' naming standard`
+    );
+
+    // 2. YAML Frontmatter Boundary check
+    assert.match(
+      content,
+      /^---\n([\s\S]*?)\n---/,
+      `Post '${file}' must start with valid YAML frontmatter delimiters`
+    );
+
+    // Extract frontmatter block
+    const fmMatch = content.match(/^---\n([\s\S]*?)\n---/);
+    const fmText = fmMatch[1];
+
+    // 3. Title Invariant
+    assert.match(
+      fmText,
+      /title:\s*"[^"]{5,}"/,
+      `Post '${file}' must contain a valid title of at least 5 characters`
+    );
+
+    // 4. Date Invariant (ISO Format)
+    assert.match(
+      fmText,
+      /date:\s*\d{4}-\d{2}-\d{2}/,
+      `Post '${file}' must define an ISO date (YYYY-MM-DD)`
+    );
+
+    // 5. Layout Invariant
+    assert.match(
+      fmText,
+      /layout:\s*layouts\/post\.njk/,
+      `Post '${file}' must specify 'layout: layouts/post.njk'`
+    );
+
+    // 6. Tags Invariant
+    assert.match(
+      fmText,
+      /tags:\s*\n(\s+-\s+[a-z0-9-]+\n)+/,
+      `Post '${file}' must specify a non-empty YAML tags list`
+    );
+
+    // 7. Excerpt Invariant
+    assert.match(
+      fmText,
+      /excerpt:\s*"[^"]{20,}"/,
+      `Post '${file}' must provide a substantive excerpt of at least 20 characters`
+    );
+  }
+});