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/database/error.go

@@ -0,0 +1,27 @@
+package database
+
+import (
+	"fmt"
+)
+
+// Error should be used for errors involving queries ran against the database
+type Error struct {
+	// Optional: the line number
+	Line uint
+
+	// Query is a query excerpt
+	Query []byte
+
+	// Err is a useful/helping error message for humans
+	Err string
+
+	// OrigErr is the underlying error
+	OrigErr error
+}
+
+func (e Error) Error() string {
+	if len(e.Err) == 0 {
+		return fmt.Sprintf("%v in line %v: %s", e.OrigErr, e.Line, e.Query)
+	}
+	return fmt.Sprintf("%v in line %v: %s (details: %v)", e.Err, e.Line, e.Query, e.OrigErr)
+}