docs(content): add documentation for content directory structure and collections

src/assets/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# assets
+
+## Purpose
+Build-time assets imported by source code (processed by Astro/Vite), as opposed to `public/` which is served verbatim.
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `roboto-mono-regular.ttf` | Font (weight 400) embedded into Satori-generated OG images. |
+| `roboto-mono-700.ttf` | Font (weight 700) for OG image titles. |
+
+## For AI Agents
+
+### Working In This Directory
+- These fonts are imported as raw buffers in `src/pages/og-image/[...slug].png.ts` (via the `vite-plugin-raw-fonts` config in `astro.config.ts`). Renaming requires updating that import.
+
+## Dependencies
+
+### Internal
+- `src/pages/og-image/` — consumes these fonts.
+
+<!-- MANUAL: -->