docs: add renderer agent workflow guide

.clintonrc.json

@@ -1,9 +1,9 @@
 {
-  "ignores": [],
+  "ignores": ["AGENTS.md"],
   "rules": {
     "pkg-main": "off",
     "ava": "off",
     "xo": "off",
     "use-travis": "off"
   }
-}
+}

AGENTS.md

@@ -0,0 +1,44 @@
+# Working on posthtml-render
+
+## Scope and workspace
+
+This repository serializes PostHTML trees. Read `package.json`, `readme.md`, `src/index.ts` and the existing tests before making behavior changes. Inspect Git status and preserve existing work.
+
+For cross-package changes, also read `../posthtml/AGENTS.md` when available, or `https://github.com/posthtml/posthtml/blob/master/AGENTS.md`. The core, parser and renderer are separate Git repositories; do not mix their changes or releases.
+
+## Current toolchain
+
+This package still uses its legacy AVA/XO/clinton and tsup toolchain. Its existing CI matrix is Node.js 12/14/16, with a published runtime minimum of Node.js 12. Use the existing CI environment to reproduce baseline toolchain failures; do not assume the modern core/parser scripts exist here.
+
+Commands from this repository's manifest:
+
+```sh
+npm ci
+npm run build
+npm test
+```
+
+`prepare` builds on install. `npm test` first runs the `pretest` clinton command, then XO and the AVA suite with coverage. Record pre-existing install/lint/test failures separately from a renderer behavior regression. Modernizing these tools is a separate scoped change and must not silently change the public runtime minimum.
+
+The current baseline has a Clinton `pkg-schema` failure resolving external SchemaStore references, reproduced on unmodified master `47e750b` with Node.js 12. This blocks the later functional checks. Keep that tooling failure distinct from changes to rendering behavior; do not disable the schema rule merely to hide it. The conventional uppercase `AGENTS.md` filename is exempted through Clinton's file ignore list.
+
+## Files and contracts
+
+- `src/index.ts`: serializer, quoting, attribute rules, void tags and option enums.
+- `test/test-core.spec.ts` and `test/templates`: exact output expectations.
+- `ava.config.js`, `xo.config.js`, `.c8rc`: current test/lint/coverage configuration.
+- Built JavaScript and declarations are generated into ignored `dist/`.
+
+Preserve raw text/comment/directive strings, numeric content, nested arrays, boolean attributes, `tag: false` unwrapping and all single-tag closing modes. The renderer currently normalizes some node content in place; test observable behavior before changing this.
+
+Quoting is a public contract: check quote styles, embedded quotes, JSON-valued attributes and unquoted values. `is-json` has specific object/array recognition semantics; a generic `JSON.parse` success check is not automatically equivalent. Compare exact rendered HTML when changing this path.
+
+Custom single-tag string names are matched against a lowercased tag name. Test mixed-case SVG/XML and RegExp tags explicitly when changing single-tag logic.
+
+## Types, integration and releases
+
+Parser is currently a development dependency used for AST declarations, not a runtime parser dependency. Preserve that distinction. Updating to newer parser declarations may require a newer development compiler without requiring a higher renderer runtime minimum; verify the emitted JavaScript and actual runtime dependencies.
+
+For a behavior change, run renderer tests, build the package, then `npm pack --ignore-scripts`. From core, run `npm run test:packed -- <core.tgz> --render <render.tgz>` and add `--parser <parser.tgz>` for coordinated AST changes. Test parser-to-renderer round trips and all core processing modes with actual tarballs rather than `npm link`.
+
+Keep runtime compatibility explicit and test the supported minimum. Review `.d.ts` compatibility against the parser version consumed by core. A documentation-only change needs no npm release. When a renderer release is requested, release it before updating the core range, and verify the published package and tag before reporting completion.