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-migrate/migrate/v4/SECURITY.md

@@ -0,0 +1,16 @@
+# Security Policy
+
+## Supported Versions
+
+| Version | Supported          |
+| ------- | ------------------ |
+| master  | :white_check_mark: |
+| 4.x     | :white_check_mark: |
+| 3.x     | :x:                |
+| < 3.0   | :x:                |
+
+## Reporting a Vulnerability
+
+We prefer [coordinated disclosures](https://en.wikipedia.org/wiki/Coordinated_vulnerability_disclosure). To start one, create a GitHub security advisory following [these instructions](https://docs.github.com/en/code-security/security-advisories/guidance-on-reporting-and-writing/privately-reporting-a-security-vulnerability)
+
+Please suggest potential impact and urgency in your reports.