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

include/kappa/dsl/ast.hpp

@@ -93,4 +93,14 @@ struct PackageDef {
     Phase uninstall;
 };
 
+struct IndexEntry {
+    std::string name;
+    std::string version;
+};
+
+struct IndexDef {
+    std::string name;                      // "kappa-os/stable"
+    std::vector<IndexEntry> packages;
+};
+
 } // namespace kappa::dsl

include/kappa/dsl/system.hpp

@@ -57,9 +57,18 @@ struct ServiceRef {
     std::unordered_map<std::string, std::string> config;
 };
 
+struct RepoDef {
+    std::string name;
+    std::string url;
+    std::vector<std::string> channels;
+    std::vector<std::string> mirrors;
+    int priority = 50;
+};
+
 struct SystemConfig {
     std::vector<std::string> imports;
-    std::vector<std::string> remotes;
+    std::vector<std::string> remotes;       // legacy — flat URL list
+    std::vector<RepoDef> repos;             // named repos with channels/mirrors
     SystemBlock system;
     std::vector<PackageRef> packages;
     std::vector<ServiceRef> services;
@@ -72,4 +81,7 @@ struct SystemConfig {
 SystemConfig parse_system_config(std::string_view source);
 SystemConfig resolve_imports(const SystemConfig& cfg, const std::string& base_dir);
 
+IndexDef parse_index(std::string_view source);
+IndexDef build_index(const std::string& directory);
+
 } // namespace kappa::dsl

include/kappa/dsl/token.hpp

@@ -49,6 +49,10 @@ enum class TokenType {
     KwUninstall,
     KwTrue,
     KwFalse,
+
+    // Repo / index
+    KwIndex,
+    KwRepos,
 };
 
 struct Token {

include/kappa/tools/format.hpp

@@ -9,5 +9,6 @@ namespace kappa::tools {
 
 void format_package(std::ostream& os, const dsl::PackageDef& pkg);
 void format_config(std::ostream& os, const dsl::SystemConfig& cfg);
+void format_index(std::ostream& os, const dsl::IndexDef& idx);
 
 } // namespace kappa::tools