docs: add AGENTS.md with plugin-specific agent guide

AGENTS.md

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+# Agent Guide — ep_reference
+
+Reference content from other Pads.
+
+## Tech stack
+
+* Etherpad plugin framework (hooks declared in `ep.json`)
+* EJS templates rendered server-side via `eejsBlock_*` hooks
+* `ep_plugin_helpers` for shared boilerplate
+
+## Project structure
+
+```
+ep_reference/
+├── AGENTS.md
+├── CONTRIBUTING.md
+├── LICENSE.md
+├── ep.json
+├── index.js
+├── package.json
+├── static/
+│   ├── css/
+│   ├── js/
+│   ├── tests/
+├── templates/
+│   ├── reference.ejs
+```
+
+## Helpers used
+
+* `pad-toggle` from `ep_plugin_helpers`
+* `pad-toggle-server` from `ep_plugin_helpers`
+
+
+## Helpers NOT used
+
+_To be audited in the helpers-adoption sweep (Phase 4)._
+
+
+## Running tests locally
+
+`ep_reference` runs inside Etherpad's test harness. From an etherpad checkout that has installed this plugin via `pnpm run plugins i --path ../ep_reference`:
+
+```bash
+# Backend (Mocha) — harness boots its own server
+pnpm --filter ep_etherpad-lite run test
+
+# Playwright — needs `pnpm run dev` in a second terminal
+pnpm --filter ep_etherpad-lite run test-ui
+```
+
+## Standing rules for agent edits
+
+* PRs target `main`. Linear commits, no merge commits.
+* Every bug fix includes a regression test in the same commit.
+* All user-facing strings in `locales/`. No hardcoded English in templates.
+* No hardcoded `aria-label` on icon-only controls — etherpad's html10n auto-populates `aria-label` from the localized string when (a) the element has a `data-l10n-id` and (b) no author-supplied `aria-label` is present. Adding a hardcoded English `aria-label` blocks that and leaves it untranslated. (See `etherpad-lite/src/static/js/vendors/html10n.ts:665-678`.)
+* No nested interactive elements (no `<button>` inside `<a>`).
+* LLM/Agent contributions are explicitly welcomed by maintainers.
+
+## Quick reference: hooks declared in `ep.json`
+
+* Server: `loadSettings`, `clientVars`, `eejsBlock_body`, `eejsBlock_mySettings`, `eejsBlock_padSettings`, `eejsBlock_dd_help`
+* Client: `postAceInit`, `handleClientMessage_CUSTOM`, `handleClientMessage_CLIENT_MESSAGE`, `aceInitialized`, `aceAttribsToClasses`, `aceDomLineProcessLineAttributes`, `aceRegisterBlockElements`, `aceEditorCSS`, `aceCreateDomLine`, `aceGetFilterStack`, `aceRegisterLineAttributes`
+
+When adding a hook, register it in both `ep.json` *and* the matching `exports.<hook> = ...` in the JS file.