Add AGENTS.md for Claude Code, Gemini CLI and Codex (#1638)

Adds AGENTS.md for Claude Code, Gemini CLI and Codex.

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Adds AGENTS.md with clear guidelines for working in this Electron +
React repo. It standardizes IPC patterns and React/TanStack Query usage
to keep code consistent and secure.

- **New Features**
- Documents IPC architecture: renderer IpcClient, preload allowlist,
host handlers, and throwing errors on failure.
- Defines React integration: useQuery for reads, useMutation for writes,
invalidate related queries, optional global state sync.
- Notes security practices: avoid remote, validate/lock by appId on
mutations.
- Encourages descriptive names that mirror IPC channels and colocated
tests/stories.

<!-- End of auto-generated description by cubic. -->

AGENTS.md

@@ -0,0 +1,25 @@
+# Repository Agent Guide
+
+## Project context
+- This is an Electron application with a secure IPC boundary.
+- Frontend is a React app that uses TanStack Router (not Next.js or React Router).
+- Data fetching/mutations should be handled with TanStack Query when touching IPC-backed endpoints.
+
+## IPC architecture expectations
+1. `src/ipc/ipc_client.ts` runs in the renderer. Access it via `IpcClient.getInstance()` and expose dedicated methods per IPC channel.
+2. `src/preload.ts` defines the renderer allowlist. New IPC APIs must be added here.
+3. `src/ipc/ipc_host.ts` registers handlers that live in files under `src/ipc/handlers/` (e.g., `app_handlers.ts`, `chat_stream_handlers.ts`, `settings_handlers.ts`).
+4. IPC handlers should `throw new Error("...")` on failure instead of returning `{ success: false }` style payloads.
+
+## React + IPC integration pattern
+When creating hooks/components that call IPC handlers:
+- Wrap reads in `useQuery`, providing a stable `queryKey`, async `queryFn` that calls the relevant `IpcClient` method, and conditionally use `enabled`/`initialData`/`meta` as needed.
+- Wrap writes in `useMutation`; validate inputs locally, call the IPC client, and invalidate related queries on success. Use shared utilities (e.g., toast helpers) in `onError`.
+- Synchronize TanStack Query data with any global state (like Jotai atoms) via `useEffect` only if required.
+
+## General guidance
+- Favor descriptive module/function names that mirror IPC channel semantics.
+- Keep Electron security practices in mind (no `remote`, validate/lock by `appId` when mutating shared resources).
+- Add tests in the same folder tree when touching renderer components.
+
+Use these guidelines whenever you work within this repository.