docs(agents): add AGENTS.md documentation files for site agents

src/utils/AGENTS.md

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+<!-- Parent: ../AGENTS.md -->
+<!-- Generated: 2026-06-30 | Updated: 2026-06-30 -->
+
+# utils
+
+## Purpose
+Pure, framework-agnostic helper functions used across components, pages, and data helpers.
+
+## Key Files
+| File | Description |
+|------|-------------|
+| `date.ts` | `getFormattedDate` (locale-aware, per `siteConfig.date`) and `collectionDateSort` — sorts `writing`/`projects`/`speaking`/etc. by `publishDate` descending. |
+| `generateToc.ts` | Builds a nested table-of-contents tree from Astro markdown headings (used by `TOC.astro`). |
+| `remark.ts` | Shared remark/markdown helpers. |
+| `domElement.ts` | Small DOM helpers for client-side scripts. |
+| `webmentions.ts` | Fetches and caches webmentions from webmention.io for the webmention components. |
+
+## For AI Agents
+
+### Working In This Directory
+- `collectionDateSort` is typed for entries with a `publishDate`; experience entries (which use `startDate`/`endDate`) are sorted in `@/data/experience.ts` instead.
+- Keep these pure and side-effect-free where possible; `webmentions.ts` is the exception (network + cache).
+
+## Dependencies
+
+### Internal
+- `@/site.config` (date locale), `@/types`.
+
+<!-- MANUAL: -->