feat: add hm-options package for Home Manager options

Add a new MCP server for Home Manager options, mirroring the
functionality of nixos-options but targeting the home-manager
repository.

Changes:
- Add shared options.Indexer interface for both implementations
- Add internal/homemanager package with indexer and channel aliases
- Add cmd/hm-options CLI entry point
- Parameterize MCP server with ServerConfig for name/instructions
- Parameterize nix/package.nix for building both packages
- Add hm-options package and NixOS module to flake.nix
- Add nix/hm-options-module.nix for systemd deployment

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

cmd/nixos-options/main.go

@@ -197,7 +197,8 @@ func runServe(c *cli.Context) error {
 	}
 
 	logger := log.New(os.Stderr, "[mcp] ", log.LstdFlags)
-	server := mcp.NewServer(store, logger)
+	config := mcp.DefaultNixOSConfig()
+	server := mcp.NewServer(store, logger, config)
 
 	indexer := nixos.NewIndexer(store)
 	server.RegisterHandlers(indexer)