feat: MCP tools and nixpkgs indexer

- Add options.json parser with mdDoc support
- Add nixpkgs indexer using nix-build
- Implement all MCP tool handlers:
  - search_options: Full-text search with filters
  - get_option: Option details with children
  - get_file: Fetch file contents
  - index_revision: Build and index options
  - list_revisions: Show indexed versions
  - delete_revision: Remove indexed data
- Add parser tests

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

internal/nixos/parser_test.go

@@ -0,0 +1,154 @@
+package nixos
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestParseOptions(t *testing.T) {
+	// Sample options.json content
+	input := `{
+		"services.nginx.enable": {
+			"declarations": ["/nix/store/xxx-source/nixos/modules/services/web-servers/nginx/default.nix"],
+			"default": false,
+			"description": "Whether to enable Nginx Web Server.",
+			"readOnly": false,
+			"type": "boolean"
+		},
+		"services.nginx.package": {
+			"declarations": ["/nix/store/xxx-source/nixos/modules/services/web-servers/nginx/default.nix"],
+			"default": {},
+			"description": {"_type": "mdDoc", "text": "The nginx package to use."},
+			"readOnly": false,
+			"type": "package"
+		},
+		"services.caddy.enable": {
+			"declarations": ["/nix/store/xxx-source/nixos/modules/services/web-servers/caddy/default.nix"],
+			"default": false,
+			"description": "Enable Caddy web server",
+			"readOnly": false,
+			"type": "boolean"
+		}
+	}`
+
+	options, err := ParseOptions(strings.NewReader(input))
+	if err != nil {
+		t.Fatalf("ParseOptions failed: %v", err)
+	}
+
+	if len(options) != 3 {
+		t.Errorf("Expected 3 options, got %d", len(options))
+	}
+
+	// Check nginx.enable
+	opt := options["services.nginx.enable"]
+	if opt == nil {
+		t.Fatal("Expected services.nginx.enable option")
+	}
+	if opt.Type != "boolean" {
+		t.Errorf("Type = %q, want boolean", opt.Type)
+	}
+	if opt.Description != "Whether to enable Nginx Web Server." {
+		t.Errorf("Description = %q", opt.Description)
+	}
+	if opt.Default != "false" {
+		t.Errorf("Default = %q, want false", opt.Default)
+	}
+	if len(opt.Declarations) != 1 {
+		t.Errorf("Expected 1 declaration, got %d", len(opt.Declarations))
+	}
+	// Check declaration path normalization
+	if !strings.HasPrefix(opt.Declarations[0], "nixos/") {
+		t.Errorf("Declaration path not normalized: %q", opt.Declarations[0])
+	}
+
+	// Check nginx.package (mdDoc description)
+	opt = options["services.nginx.package"]
+	if opt == nil {
+		t.Fatal("Expected services.nginx.package option")
+	}
+	if opt.Description != "The nginx package to use." {
+		t.Errorf("Description = %q (mdDoc not extracted)", opt.Description)
+	}
+}
+
+func TestNormalizeDeclarationPath(t *testing.T) {
+	tests := []struct {
+		input  string
+		expect string
+	}{
+		{
+			"/nix/store/xxx-source/nixos/modules/services/web-servers/nginx/default.nix",
+			"nixos/modules/services/web-servers/nginx/default.nix",
+		},
+		{
+			"/nix/store/xxx/pkgs/top-level/all-packages.nix",
+			"pkgs/top-level/all-packages.nix",
+		},
+		{
+			"/nix/store/abc123/lib/types.nix",
+			"lib/types.nix",
+		},
+		{
+			"relative/path.nix",
+			"relative/path.nix",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			got := normalizeDeclarationPath(tt.input)
+			if got != tt.expect {
+				t.Errorf("normalizeDeclarationPath(%q) = %q, want %q", tt.input, got, tt.expect)
+			}
+		})
+	}
+}
+
+func TestExtractDescription(t *testing.T) {
+	tests := []struct {
+		name   string
+		input  interface{}
+		expect string
+	}{
+		{
+			"string",
+			"Simple description",
+			"Simple description",
+		},
+		{
+			"mdDoc",
+			map[string]interface{}{
+				"_type": "mdDoc",
+				"text":  "Markdown description",
+			},
+			"Markdown description",
+		},
+		{
+			"description key",
+			map[string]interface{}{
+				"description": "Nested description",
+			},
+			"Nested description",
+		},
+		{
+			"nil",
+			nil,
+			"",
+		},
+		{
+			"empty map",
+			map[string]interface{}{},
+			"",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := extractDescription(tt.input)
+			if got != tt.expect {
+				t.Errorf("extractDescription = %q, want %q", got, tt.expect)
+			}
+		})
+	}
+}