docs: add a contributing guide

CONTRIBUTING.md

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+# Contributing to CoreTrace Desktop
+
+Thanks for your interest. This guide takes you from choosing an issue to a merged pull request.
+
+## Find something to work on
+
+- Issues labelled
+  [`good first issue`](https://github.com/CoreTrace/coretrace-gui/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22good%20first%20issue%22)
+  are scoped for a first contribution.
+- Comment on the issue to ask for it; a maintainer assigns it to you and answers questions
+  there.
+- For a larger change, open an issue first to agree on the approach.
+- Report a bug with the steps to reproduce it, your operating system and the version shown in
+  the About panel. For a vulnerability, follow [SECURITY.md](SECURITY.md) instead of opening a
+  public issue.
+
+## Set up
+
+Install Node.js 22.19 or newer, Rust 1.95 or newer and the
+[Tauri prerequisites](https://v2.tauri.app/start/prerequisites/) of your platform:
+
+- **Linux** (Ubuntu or Debian), the packages CI installs:
+  `sudo apt-get install libwebkit2gtk-4.1-dev libappindicator3-dev librsvg2-dev patchelf libssl-dev libdbus-1-dev`
+- **Windows**: Microsoft C++ Build Tools and the WebView2 Runtime.
+- **macOS**: the Xcode Command Line Tools.
+
+```sh
+npm ci
+npm start      # the native application
+npm run dev    # browser preview: no filesystem, Git or account actions
+```
+
+## Where the code is
+
+| Path | Content |
+|---|---|
+| `desktop/` | React and TypeScript interface, typed native bridge, and its tests (`*.test.ts`, `*.test.tsx`) |
+| `src-tauri/src/` | Rust services: workspace, GitHub, cloud account and API, local analysis |
+| `desktop/platform-schema.d.ts` | Generated from the cloud platform's API contract: do not edit it by hand |
+| `src/`, `tests/` | The old Electron implementation, kept as a reference: do not change it |
+
+The interface is in French: write new labels and messages in French.
+
+## Before you open a pull request
+
+Run what CI runs, on Windows, macOS and Linux:
+
+```sh
+npm run check            # TypeScript types
+npm run format:check     # Prettier; `npm run format` fixes it
+npm test                 # interface tests (Vitest)
+cargo fmt --manifest-path src-tauri/Cargo.toml --check
+cargo clippy --manifest-path src-tauri/Cargo.toml --all-targets -- -D warnings
+npm run test:native      # Rust tests
+```
+
+Add or update the tests next to the code you change.
+
+## Pull request
+
+- Branch from `main` and name the branch after the issue, `<number>-<short-description>`, for
+  example `85-fix-breadcrumb-contrast`. The "Create a branch" link of the issue does it for you.
+- Write commit subjects as English Conventional Commits with a scope, for example
+  `fix(desktop): raise the contrast of the breadcrumb`.
+- In the description, say what changed and why, link the issue (`Closes #85`), list the commands
+  you ran, and add a screenshot for any visible change.
+- CI must pass on the three systems before a maintainer reviews and merges.
+
+Contributions are licensed under Apache-2.0, like the project (section 5 of the
+[license](LICENSE)). This project follows the CoreTrace
+[Code of Conduct](https://github.com/CoreTrace/coretrace/blob/main/CODE_OF_CONDUCT.md).