feat: implement NATS-based NixOS deployment system

Implement the complete homelab-deploy system with three operational modes:

- Listener mode: Runs on NixOS hosts as a systemd service, subscribes to
  NATS subjects with configurable templates, executes nixos-rebuild on
  deployment requests with concurrency control

- MCP mode: MCP server exposing deploy, deploy_admin, and list_hosts
  tools for AI assistants with tiered access control

- CLI mode: Manual deployment commands with subject alias support via
  environment variables

Key components:
- internal/messages: Request/response types with validation
- internal/nats: Client wrapper with NKey authentication
- internal/deploy: Executor with timeout and lock for concurrency
- internal/listener: Subject template expansion and request handling
- internal/cli: Deploy logic with alias resolution
- internal/mcp: MCP server with mcp-go integration
- nixos/module.nix: NixOS module with hardened systemd service

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/mcp/server_test.go

@@ -0,0 +1,43 @@
+package mcp
+
+import (
+	"testing"
+	"time"
+)
+
+func TestNew(t *testing.T) {
+	cfg := ServerConfig{
+		NATSUrl:         "nats://localhost:4222",
+		NKeyFile:        "/path/to/key",
+		EnableAdmin:     false,
+		AdminNKeyFile:   "",
+		DiscoverSubject: "deploy.discover",
+		Timeout:         10 * time.Minute,
+	}
+
+	s := New(cfg)
+
+	if s == nil {
+		t.Fatal("New() returned nil")
+	}
+	if s.server == nil {
+		t.Error("server should not be nil")
+	}
+}
+
+func TestNew_WithAdmin(t *testing.T) {
+	cfg := ServerConfig{
+		NATSUrl:         "nats://localhost:4222",
+		NKeyFile:        "/path/to/key",
+		EnableAdmin:     true,
+		AdminNKeyFile:   "/path/to/admin/key",
+		DiscoverSubject: "deploy.discover",
+		Timeout:         10 * time.Minute,
+	}
+
+	s := New(cfg)
+
+	if s == nil {
+		t.Fatal("New() returned nil")
+	}
+}