Add AGENTS.md with Cursor Cloud development instructions

AGENTS.md

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+# AGENTS.md
+
+## Cursor Cloud specific instructions
+
+### Project overview
+
+PinStick is a lightweight desktop note-pinning app with two implementations:
+- **Native macOS app** — Swift/SwiftUI (cannot build on Linux)
+- **Cross-platform app** — Tauri 1.5 (Rust + HTML/CSS/JS) under `cross-platform/`
+
+Cloud agents work exclusively with the **cross-platform Tauri app** since the macOS native app requires Xcode/macOS.
+
+### Running the cross-platform app
+
+```bash
+cd cross-platform
+npm install
+npx tauri dev    # development mode with hot-reload
+npm run build    # production build (outputs .deb and .AppImage)
+npm run check    # debug build without bundling (fast compile check)
+```
+
+### Linting and formatting
+
+```bash
+cd cross-platform/src-tauri
+cargo clippy       # Rust linter
+cargo fmt --check  # Rust format check
+```
+
+There is no JavaScript linter configured (the frontend is vanilla JS with no ESLint).
+
+### Key caveats (Ubuntu 24.04 / Noble)
+
+- **webkit2gtk-4.0 is unavailable** on Ubuntu 24.04. Tauri 1.5 requires it, but only `webkit2gtk-4.1` ships with Noble. The update script installs the 4.1 packages and creates pkg-config shims (`webkit2gtk-4.0.pc`, `javascriptcoregtk-4.0.pc`) plus library symlinks (`libwebkit2gtk-4.0.so` → `libwebkit2gtk-4.1.so`, etc.) to satisfy the build.
+- **Rust stable must be the default toolchain.** The pre-installed pinned version (1.83) is too old; `rustup default stable` switches to the latest stable (1.95+), which supports `edition2024` required by transitive dependencies.
+- The Tauri dev server emits a harmless deprecation warning: `webkit_settings_set_enable_offline_web_application_cache is deprecated and does nothing.` — this is safe to ignore.
+
+### Tests
+
+There are no automated tests for the cross-platform Tauri app. The native macOS app has `JotTests.swift` and `JotUITests.swift` but these cannot run on Linux.
+
+Manual testing: launch the app with `npx tauri dev` (requires an X11 display) and verify the note textarea and pin button work.