feat: add content collection with schema and reading-time utility

- Created src/content.config.ts with Astro defineCollection using glob loader, pattern '**/*.{md,mdx}', base './src/content/posts', and Zod schema with 5 fields (title, date, type, tags, summary)
- Created src/content/posts/ directory for content storage
- Created src/utils/reading-time.ts with getReadingTime function (split whitespace, 200 WPM, Math.max(1, Math.ceil))
- All 23 new tests pass, all 61 total tests pass, npm run build succeeds

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Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

src/content.config.ts

@@ -0,0 +1,18 @@
+import { defineCollection, z } from 'astro:content';
+import { glob } from 'astro/loaders';
+
+const posts = defineCollection({
+  loader: glob({
+    pattern: '**/*.{md,mdx}',
+    base: './src/content/posts',
+  }),
+  schema: z.object({
+    title: z.string(),
+    date: z.coerce.date(),
+    type: z.enum(['post', 'pattern', 'lesson']).default('post'),
+    tags: z.array(z.string()).default([]),
+    summary: z.string().optional(),
+  }),
+});
+
+export const collections = { posts };