docs: update design with configurable subjects and improved module

- Add configurable NATS subject patterns with template variables
  (<hostname>, <tier>, <role>) for multi-tenant setups
- Add deploy.discover subject for host discovery
- Simplify CLI to use direct subjects with optional aliases via
  HOMELAB_DEPLOY_ALIAS_* environment variables
- Clarify request/reply flow with UUID-based response subjects
- Expand NixOS module with hardening options, package option,
  and configurable deploy/discover subjects
- Switch CLI framework from cobra to urfave/cli/v3

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

CLAUDE.md

@@ -40,7 +40,7 @@ Subjects follow `deploy.<tier>.<target>`:
 ### Planned Package Structure
 
 ```
-cmd/homelab-deploy/main.go    # CLI entrypoint with cobra subcommands
+cmd/homelab-deploy/main.go    # CLI entrypoint with urfave/cli subcommands
 internal/listener/            # Listener mode (NATS subscription, nixos-rebuild execution)
 internal/mcp/                 # MCP server mode
 internal/nats/                # NATS client wrapper