docs(agents): add AGENTS.md instructions for AI agents

AGENTS.md

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+# Agent Instructions for Rift
+
+Guidance for AI agents working in this repository. Rift is a tiling window manager for macOS written in Rust, focusing on performance, usability, and leveraging private APIs.
+
+## Core Commands
+
+Always run formatting and verify tests pass before reporting work as complete.
+
+```bash
+# 1. Format code using the nightly toolchain (configured in rustfmt.toml)
+cargo +nightly fmt --all
+
+# 2. Run all unit and integration tests
+cargo test
+
+# 3. Run a specific test
+cargo test <test_name_pattern>
+
+# 4. Check formatting without modifying files
+cargo +nightly fmt --all --check
+
+# 5. Build the project
+cargo build
+```
+
+## Local Git Hooks
+
+Rift uses local Git hooks stored in the repository. Once cloned, you should configure your local git client to use them:
+```bash
+git config core.hooksPath .githooks
+```
+- **Pre-commit:** Automatically runs `cargo +nightly fmt --all` and stages any formatted changes.
+- **Pre-push:** Automatically runs `cargo test` and prevents pushing if any tests fail.
+
+## Project Rules
+
+- **Strict Formatting:** Code must be formatted using the nightly Rust toolchain: `cargo +nightly fmt --all`. Settings are defined in `rustfmt.toml`.
+- **Testing:** Always write unit tests for new behavior or bug fixes. Tests are usually co-located in the module file inside a `mod tests` block.
+- **Private macOS APIs:** Rift interacts with undocumented private macOS APIs (such as Skylight, SkyLight, SLS, etc.) to control spaces, displays, and windows. Be extremely cautious when modifying these bindings.
+- **Error Handling:** Prefer `anyhow` for applications/CLI errors, and clean `Result` propagation inside the layout engine core.
+
+## Commit Messages
+
+Angular format `type(scope): description`.
+* **Types:** `feat`, `fix`, `docs`, `style`, `refactor`, `perf`, `test`, `build`, `ci`, `chore`.
+* **Examples:**
+  - `feat(layout): add master-stack layout custom settings`
+  - `fix(hotkey): handle keycode translation on ISO keyboards`
+  - `ci(githooks): add pre-commit and pre-push hooks`
+* **Rule:** **Never** use `fix(test):` or `fix(e2e):` — test changes must use `test:` type.
+
+## Anti-patterns
+
+| Avoid | Do instead |
+| --- | --- |
+| Running `cargo fmt` without `+nightly` | Use `cargo +nightly fmt` to support custom formatting rules |
+| Committing directly to `main` without formatting | Ensure the pre-commit hook is active locally via `git config core.hooksPath .githooks` |
+| Over-relying on public Cocoa APIs for workspace management | Leverage the custom private API bindings under `src/sys/` |