docs: add a security policy

SECURITY.md

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+# Security Policy
+
+## Supported versions
+
+| Version                       | Supported |
+| ----------------------------- | --------- |
+| 6.x (Tauri desktop, `main`)   | Yes       |
+| 5.x and earlier (Electron)    | No        |
+
+Only the latest 6.x pre-release receives fixes. The Electron implementation kept in
+`src/` is reference code: it is not built, shipped or patched.
+
+## Reporting a vulnerability
+
+Do not open a public issue, discussion or pull request for a vulnerability.
+
+Report it privately through GitHub:
+[Security → Report a vulnerability](https://github.com/CoreTrace/coretrace-gui/security/advisories/new).
+
+Include:
+
+- the affected version or commit, and the operating system;
+- the component (React UI, native command, cloud API client, local `ctrace` runner);
+- reproduction steps or a proof of concept;
+- the impact you observed or expect.
+
+We aim to acknowledge reports within 5 working days and to share an assessment and
+remediation plan within 15 working days. We will credit you in the advisory unless you
+ask us not to. Please give us reasonable time to release a fix before disclosing.
+
+Vulnerabilities in the CoreTrace cloud service itself (`api.coretrace.fr`,
+`app.coretrace.fr`) may be reported through the same channel; we will route them.
+
+## Scope
+
+In scope, among others:
+
+- escaping the chosen workspace when reading, writing or listing files, including
+  through links, `..` segments or Git metadata;
+- exposing access or refresh tokens to the web view, logs, `localStorage` or config files;
+- reaching native commands or the network from the web view beyond what
+  `src-tauri/capabilities/` and the Content Security Policy allow;
+- accepting non-HTTPS API, device-login or report URLs outside loopback development;
+- command or argument injection when cloning repositories or running `ctrace`;
+- tampered or unverified cloud reports being displayed as verified;
+- spending CTU without explicit user confirmation.
+
+Out of scope:
+
+- findings that require an already compromised machine or user account;
+- vulnerabilities in third-party analysers run by `ctrace`, Git, or the OS credential
+  manager (report them upstream);
+- the browser preview (`npm run dev`), which has no filesystem, Git or account access;
+- missing code signing or automatic updates: release builds are currently unsigned and
+  this is documented in the [README](README.md#build-and-verify).
+
+## Security design notes
+
+- The web view only holds the `core:default` and window-destroy permissions; workspace,
+  Git, process and network access live in Rust commands.
+- Access tokens stay in Rust memory; refresh tokens are stored in the OS credential
+  manager, keyed by API URL.
+- File edits are confined to the workspace, limited to 4 MiB UTF-8 text, and saved
+  atomically after checking the on-disk revision.
+- Reports, process output, run duration and clone duration are capped.
+- Opening or cloning a repository never builds it or installs its dependencies.