Add a security policy that points to private vulnerability reporting (#526)

Co-authored-by: Paarangat Jain <44281208+pjrus@users.noreply.github.com>

SECURITY.md

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+# Security policy
+
+## Supported versions
+
+SolidCraft is young and moves quickly, so only the latest release gets security fixes. Please
+update to the [latest release](https://github.com/bherbruck/solidcraft/releases/latest) before
+reporting, and check whether the problem is still on `main`.
+
+| Version | Supported |
+| --- | --- |
+| Latest release | ✅ |
+| Nightly builds | ✅ (fixes land here first) |
+| Older releases | ❌ |
+
+## Reporting a vulnerability
+
+Please **don't open a public issue** for a security problem. Report it privately through GitHub:
+open the repository's [Security tab](https://github.com/bherbruck/solidcraft/security) and choose
+**Report a vulnerability**
+([direct link](https://github.com/bherbruck/solidcraft/security/advisories/new)).
+
+Please include what you found, how to reproduce it (a design file, command or MCP request), the
+version and platform, and the impact you expect. I'll acknowledge the report as soon as I can,
+keep you updated in the advisory, and credit you when the fix is published unless you'd rather
+stay anonymous.
+
+Areas where reports are especially useful:
+
+- opening untrusted design, STEP, STL or 3MF files (crashes, excessive memory or CPU, path tricks)
+- the MCP server and the control API (file access outside the allowed folders, command injection)
+- the web app (anything reaching beyond the page's own storage)
+- release packages and the update paths (AppImage, Flatpak, MSI, deb, rpm, snap)