Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/golang-jwt/jwt/v5/SECURITY.md

@@ -0,0 +1,19 @@
+# Security Policy
+
+## Supported Versions
+
+As of November 2024 (and until this document is updated), the latest version `v5` is supported. In critical cases, we might supply back-ported patches for `v4`.
+
+## Reporting a Vulnerability
+
+If you think you found a vulnerability, and even if you are not sure, please report it a [GitHub Security Advisory](https://github.com/golang-jwt/jwt/security/advisories/new). Please try be explicit, describe steps to reproduce the security issue with code example(s).
+
+You will receive a response within a timely manner. If the issue is confirmed, we will do our best to release a patch as soon as possible given the complexity of the problem.
+
+## Public Discussions
+
+Please avoid publicly discussing a potential security vulnerability.
+
+Let's take this offline and find a solution first, this limits the potential impact as much as possible.
+
+We appreciate your help!