feat: add repos DSL, index format, and kappa index command

- Add repos { } block to system config with named repos, channels,
  mirrors, and priority

- Add index "name" { } file format for package indexes

- Add kappa index <dir> subcommand that scans .kap files and
  generates index.kap

- parse_index() and build_index() in DSL module

- format_index() roundtrip support in tools module

- validate and format subcommands handle all three formats
  (package, config, index) with automatic detection

- Backward compatible: remotes = [...] still works unchanged

src/dsl/lexer.cpp

@@ -33,6 +33,8 @@ static const std::unordered_map<std::string_view, TokenType> keywords = {
     {"uninstall", TokenType::KwUninstall},
     {"true",     TokenType::KwTrue},
     {"false",    TokenType::KwFalse},
+    {"index",    TokenType::KwIndex},
+    {"repos",    TokenType::KwRepos},
 };
 
 std::string_view token_name(TokenType type) {
@@ -74,6 +76,8 @@ std::string_view token_name(TokenType type) {
     case TokenType::KwUninstall: return "uninstall";
     case TokenType::KwTrue:     return "true";
     case TokenType::KwFalse:    return "false";
+    case TokenType::KwIndex:    return "index";
+    case TokenType::KwRepos:    return "repos";
     }
     return "?";
 }