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

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## 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.

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