docs(docs): add AGENTS documentation and README files

src/layouts/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# layouts
+
+## Purpose
+Page shells that wrap route content with `<head>`, header, footer, and theming.
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `Base.astro` | Root shell for all pages. Renders `BaseHead`, `ThemeProvider`, `Header`, `<main class="mb-20">`, `Footer`, and prefetch speculation rules. Takes a `meta: SiteMeta` prop; `description` defaults to `siteConfig.description`. Sets `max-w-3xl`, `font-mono`, padding. |
+| `BlogPost.astro` | Layout for individual writing entries. Renders `Masthead`, optional `TOC`, the prose body, webmentions, and a back-to-top button. Builds the OG image URL as `/og-image/${post.id}.png`. |
+
+## For AI Agents
+
+### Working In This Directory
+- Every page should render inside `Base.astro` (directly or via `BlogPost.astro`) and pass a `meta` object (`title` required; `description`, `ogImage`, `articleDate` optional).
+- `<main>` carries `mb-20` to separate content from the footer's top border — adjust here for global content/footer spacing.
+
+## Dependencies
+
+### Internal
+- `@/components/BaseHead.astro`, `@/components/layout/*`, `@/components/blog/*`, `@/site.config`, `@/types`.
+
+<!-- MANUAL: -->