docs: add changelog update instructions to AGENTS.md

AGENTS.md

@@ -53,3 +53,13 @@ TERM=dumb cargo build --features server  # With server feature
 - Use `html_escape` crate for all user-controlled data in HTML pages
 - `esc()` for text content, `esc_attr()` for HTML attributes
 - Security headers middleware: `X-Content-Type-Options: nosniff`, `X-Frame-Options: DENY`, `Referrer-Policy: strict-origin-when-cross-origin`
+
+## Changelog
+
+The project uses [Keep a Changelog](https://keepachangelog.com/). The changelog lives at `CHANGELOG.md` in the project root.
+
+- **Always update `CHANGELOG.md`** when making changes that affect users (new features, breaking changes, bug fixes, etc.)
+- Add entries under the `[Unreleased]` section using these categories: `Added`, `Changed`, `Deprecated`, `Removed`, `Fixed`, `Security`
+- Keep descriptions concise and user-focused — what changed from the user's perspective, not implementation details
+- Commit changelog updates in the same commit as the feature/fix they document
+- Before releasing a new version, move `[Unreleased]` entries to a versioned section (e.g., `[0.2.0] - YYYY-MM-DD`) and add a new empty `[Unreleased]` above it