docs(ui): add AGENTS.md files for component documentation

src/components/blog/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# blog
+
+## Purpose
+Components for rendering Writing (the `writing` collection) — post mastheads, list previews, table of contents, and webmentions. (Folder is named `blog` for historical reasons; it serves the `/writing` section.)
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `Masthead.astro` | Post header: cover image, title, date, reading time, updated badge, and `ExperienceTags` (linked experience chips — replaces the old tag links). |
+| `PostPreview.astro` | One-line writing entry (date + linked title) used in lists; links to `/writing/<id>`. |
+| `TOC.astro` / `TOCHeading.astro` | Table of contents built from rendered headings. |
+
+## Subdirectories
+| Directory | Purpose |
+|-----------|---------|
+| `webmentions/` | Webmention display components (see `webmentions/AGENTS.md`). |
+
+## For AI Agents
+
+### Working In This Directory
+- These consume `CollectionEntry<"writing">`.
+- `Masthead` renders `data.experiences` via `ExperienceTags`; it no longer links freeform `tags` to a `/tags` route (that route was removed).
+
+## Dependencies
+
+### Internal
+- `@/components/ExperienceTags.astro`, `@/components/FormattedDate.astro`, `@/utils/generateToc`.
+
+<!-- MANUAL: -->