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>
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