Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

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

vendor/github.com/prometheus/procfs/.golangci.yml

@@ -0,0 +1,45 @@
+version: "2"
+linters:
+  enable:
+    - forbidigo
+    - godot
+    - misspell
+    - revive
+    - testifylint
+  settings:
+    forbidigo:
+      forbid:
+        - pattern: ^fmt\.Print.*$
+          msg: Do not commit print statements.
+    godot:
+      exclude:
+        # Ignore "See: URL".
+        - 'See:'
+      capital: true
+    misspell:
+      locale: US
+  exclusions:
+    generated: lax
+    presets:
+      - comments
+      - common-false-positives
+      - legacy
+      - std-error-handling
+    paths:
+      - third_party$
+      - builtin$
+      - examples$
+formatters:
+  enable:
+    - gofmt
+    - goimports
+  settings:
+    goimports:
+      local-prefixes:
+        - github.com/prometheus/procfs
+  exclusions:
+    generated: lax
+    paths:
+      - third_party$
+      - builtin$
+      - examples$