feat: add index-aware recipe resolution with repo support

- fetch_recipe_from_repos() iterates repos by priority, channels, with
  mirror fallback — caches indexes at /cache/indexes/

- Conditional HTTP downloads (curl -z) to only re-fetch stale indexes

- build_registry and fetch-package prefer repos { } over remotes = [...]
  when both are present

- Fallback chain: local .kap → repos (index-aware) → remotes (legacy)

src/main.cpp

@@ -132,7 +132,26 @@ static resolve::Registry build_registry(const dsl::SystemConfig& cfg) {
             }
         }
 
-        // If not found locally, try remotes
+        // If not found locally, try repos (index-aware) first, then legacy remotes
+        if (!found && !cfg.repos.empty()) {
+            auto result = fetch::fetch_recipe_from_repos(pref.name, cfg.repos);
+            if (result.ok && !result.path.empty()) {
+                std::ifstream in(result.path);
+                if (in) {
+                    std::ostringstream buf;
+                    buf << in.rdbuf();
+                    try {
+                        auto pkg = dsl::parse(buf.str());
+                        registry[pkg.name] = std::move(pkg);
+                        found = true;
+                    } catch (const std::exception& e) {
+                        std::cerr << "warning: parse error in remote recipe " << pref.name << ": " << e.what() << "\n";
+                    } catch (...) {
+                        std::cerr << "warning: unknown parse error in remote recipe " << pref.name << "\n";
+                    }
+                }
+            }
+        }
         if (!found && !cfg.remotes.empty()) {
             auto result = fetch::fetch_recipe(pref.name, cfg.remotes);
             if (result.ok && !result.path.empty()) {
@@ -308,19 +327,26 @@ int main(int argc, char* argv[]) {
         }
         try {
             std::vector<std::string> remotes;
+            std::vector<dsl::RepoDef> repos;
             auto config_path = std::filesystem::path(paths::system_dir()) / "config.kap";
             if (std::filesystem::exists(config_path)) {
                 auto cfg_src = read_file(config_path.c_str());
                 try {
                     auto cfg = dsl::parse_system_config(cfg_src);
                     remotes = cfg.remotes;
+                    repos = std::move(cfg.repos);
                 } catch (const std::exception& e) {
                     std::cerr << "warning: config parse error: " << e.what() << "\n";
                 } catch (...) {
                     std::cerr << "warning: unknown config parse error\n";
                 }
             }
-            auto result = fetch::fetch_recipe(file_arg, remotes);
+            fetch::RecipeResult result;
+            if (!repos.empty()) {
+                result = fetch::fetch_recipe_from_repos(file_arg, repos);
+            } else {
+                result = fetch::fetch_recipe(file_arg, remotes);
+            }
             if (result.ok) {
                 if (result.updated) {
                     std::cout << "fetched " << file_arg << " " << result.version