[Repo Assist] docs: add SECURITY.md with vulnerability reporting policy and security model

SECURITY.md

@@ -0,0 +1,28 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| 0.0.9   | ✅ Current release |
+| < 0.0.9 | ❌ No longer supported |
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability in this extension, please report it responsibly by opening a [GitHub issue](https://github.com/soderlind/vscode-phpcbf/issues/new) with the title prefixed `[Security]`. For particularly sensitive vulnerabilities you can instead contact the maintainer directly via GitHub.
+
+We aim to acknowledge reports within **3 business days** and provide a fix or mitigation within **30 days** for confirmed vulnerabilities.
+
+## Security Model
+
+This extension spawns the external `phpcbf` binary as a child process to format PHP files. Its security properties:
+
+- **Process spawning**: The executable path is taken from the VS Code setting `phpcbf.executablePath`. Only the maintainer/administrator who controls workspace settings can alter this path. An attacker who can write to your workspace's `.vscode/settings.json` can point `executablePath` at an arbitrary executable — treat that file accordingly.
+
+- **Temporary files**: The extension writes each formatted document to a randomly-named temp file (in `os.tmpdir()`), runs phpcbf on it, reads the result back, and immediately deletes the temp file. No file content is sent over a network.
+
+- **No network access**: The extension itself makes no outbound network connections. All processing is local.
+
+- **Standard path**: The `phpcbf.standard` and `phpcbf.configSearch` settings control which coding standard phpcbf uses. A malicious `phpcs.xml` file in the workspace could potentially influence phpcbf's behaviour; the same caution applies as with any project-level tool configuration.
+
+In summary, the extension's attack surface is limited to local files and process execution; it has no credentials, no secrets, and no network communication of its own.