chore(agents): add AGENTS.md/CLAUDE.md rules for AI coding agents

Any AI agent working in this repo (Claude Code, Codex, Kimi, or any
other) must stay schema-driven and never hardcode object types, field
names, filters, or columns as a shortcut. AGENTS.md is the index (tech
stack, commands, universal rules); CLAUDE.md just points Claude Code at
it so the same rules apply without duplicating content. The detailed
schema-fidelity rule lives in .agents/rules/.

.gitignore previously excluded *.md except README/CHANGELOG, which
would have hidden these files (and SCHEMA_DEVIATIONS.md) from git.

AGENTS.md

@@ -0,0 +1,28 @@
+# Stalwart WebUI Fork
+
+Community fork of [stalwartlabs/webui](https://github.com/stalwartlabs/webui), a schema-driven admin panel for [Stalwart](https://stalw.art). The server's JSON schema (fetched from `/api/schema`) is the single source of truth for forms, fields, filters, columns, and navigation.
+
+## Tech Stack
+
+- React 19 + Vite + TypeScript, Zustand stores, JMAP (RFC 8620) for all data operations.
+
+## Commands
+
+- `npm run dev` - Dev server
+- `npm run typecheck` - `tsc --noEmit`
+- `npm run lint` - ESLint
+- `npm run test` - Vitest (`npm run test:watch` to watch)
+- `npm run build` - `tsc -b && vite build`
+
+## Rules
+
+The detailed rules live in `.agents/rules/`. Read the relevant file before acting:
+
+- **Schema fidelity** - [.agents/rules/schema-fidelity.md](.agents/rules/schema-fidelity.md) - Stay schema-driven; how to handle cases the schema can't cover yet
+
+## Universal Rules
+
+- This applies to every AI coding agent working in this repo (Claude Code, Codex, Kimi, or any other) — not just one tool.
+- Never hardcode object types, field names, filters, or columns as a shortcut. Read `.agents/rules/schema-fidelity.md` before adding anything that touches lists, forms, or navigation.
+- **CRITICAL**: `src/types/schema.ts` mirrors the official server/webui schema contract and must never be edited to accommodate a deviation, not even to add an optional field. Deviation-only type augmentations go in `src/lib/schemaDeviationTypes.ts` (or the deviation's own module) as an intersection with the official type — see `.agents/rules/schema-fidelity.md`.
+- Any client-side workaround for something the official schema doesn't support yet must be documented in `SCHEMA_DEVIATIONS.md` and tagged `// SCHEMA-DEVIATION: <id>` in code. Never add one silently.