docs(docs): add documentation for public assets directory

public/AGENTS.md

@@ -0,0 +1,22 @@
+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# public
+
+## Purpose
+Static assets copied verbatim to the site root at build time (served at `/`).
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `avatar.png` | Personal avatar used as the header logo (`src/components/layout/Header.astro`, referenced as `/avatar.png`) and in the homepage `Person` JSON-LD. |
+| `icon.svg` | Source icon; `astro-webmanifest` generates favicons / apple-touch / manifest icons from it at build (referenced in `BaseHead.astro`). |
+| `social-card.png` | Default Open Graph image when a page/post supplies no `ogImage`. |
+
+## For AI Agents
+
+### Working In This Directory
+- Reference these with absolute root paths (`/avatar.png`), not the `@/` alias — they are not processed by Astro's asset pipeline.
+- Replacing `icon.svg` changes all generated favicons/manifest icons; keep it roughly square (build warns otherwise).
+
+<!-- MANUAL: -->