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/go-viper/mapstructure/v2/.editorconfig

@@ -0,0 +1,24 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_size = 4
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.go]
+indent_style = tab
+
+[{Makefile,*.mk}]
+indent_style = tab
+
+[*.nix]
+indent_size = 2
+
+[.golangci.yaml]
+indent_size = 2
+
+[devenv.yaml]
+indent_size = 2