docs: add SECURITY.md security policy (#336)

SECURITY.md

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+# Security Policy
+
+## Reporting a vulnerability
+
+Please report security vulnerabilities **privately** — do not open a public issue,
+pull request, or discussion for a suspected vulnerability.
+
+Use GitHub's private vulnerability reporting:
+
+1. Go to the repository's **Security** tab.
+2. Click **Report a vulnerability**.
+3. Describe the issue, the affected component/version, and a reproduction if possible.
+
+This opens a private advisory visible only to the maintainers. We will acknowledge
+the report, investigate, and coordinate a fix and disclosure with you. Please give us
+reasonable time to address the issue before any public disclosure.
+
+If you are unable to use private reporting, contact a maintainer listed in the
+repository metadata and request a private channel before sharing details.
+
+## Supported versions
+
+SKaiNET is pre-1.0 and evolving quickly. Security fixes are applied to the
+**latest release** and the **`develop`** branch. Older versions are not maintained;
+please upgrade to the latest version before reporting.
+
+## Scope
+
+In scope:
+
+- The SKaiNET libraries published from this repository.
+- Memory-safety, parsing, and deserialization issues in the model I/O readers
+  (GGUF, SafeTensors, ONNX) when handling untrusted model files.
+- Issues in generated export artifacts (e.g. Minerva/StableHLO) that could lead to
+  unsafe code on a consumer's device.
+
+Out of scope:
+
+- Vulnerabilities in third-party dependencies — report those upstream (we will still
+  bump the dependency once a fix is available).
+- Denial of service from intentionally malformed inputs where the documented
+  contract is "trusted input only."
+
+## Hardening and best practices
+
+Broader open-source security posture (REUSE/OpenSSF Best Practices, SBOM, dependency
+scanning) is tracked in the project's open-source best-practices work. See the
+[Best Practices](https://www.bestpractices.dev/) program for the criteria we are
+working toward.