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/main.cpp

@@ -52,6 +52,7 @@ Subcommands:
   rebuild       <config>  Compare config to installed state, rebuild changed
   list                    List installed packages
   rollback                Show available generations
+  index        <dir>      Build an index.kap from .kap files in a directory
 
 Options:
   -h, --help              Show this help message
@@ -196,8 +197,9 @@ int main(int argc, char* argv[]) {
                          || (subcommand == "build")
                          || (subcommand == "rebuild")
                          || (subcommand == "list")
-                          || (subcommand == "fetch-package")
-                          || (subcommand == "rollback");
+                           || (subcommand == "fetch-package")
+                           || (subcommand == "rollback")
+                           || (subcommand == "index");
 
     if (!valid_subcommand) {
         std::cerr << "error: unknown subcommand '" << subcommand << "'\n\n";
@@ -276,6 +278,29 @@ int main(int argc, char* argv[]) {
         return 0;
     }
 
+    if (subcommand == "index") {
+        if (file_arg == nullptr) {
+            std::cerr << "error: no directory specified\n";
+            return 1;
+        }
+        try {
+            auto idx = dsl::build_index(file_arg);
+            auto out_path = std::filesystem::path(file_arg) / "index.kap";
+            std::ofstream out(out_path);
+            if (!out) {
+                std::cerr << "error: cannot write " << out_path.string() << "\n";
+                return 1;
+            }
+            tools::format_index(out, idx);
+            std::cout << idx.packages.size() << " packages indexed → "
+                      << out_path.string() << "\n";
+            return 0;
+        } catch (const std::exception& e) {
+            std::cerr << "index error: " << e.what() << "\n";
+            return 1;
+        }
+    }
+
     if (subcommand == "fetch-package") {
         if (file_arg == nullptr) {
             std::cerr << "error: no package name specified\n";
@@ -331,7 +356,8 @@ int main(int argc, char* argv[]) {
 
     if (file_arg == nullptr
         && subcommand != "list"
-        && subcommand != "rollback") {
+        && subcommand != "rollback"
+        && subcommand != "index") {
         std::cerr << "error: no input file specified\n";
         return 1;
     }
@@ -378,6 +404,12 @@ int main(int argc, char* argv[]) {
                 }
                 std::cout << "\n";
             }
+            if (!cfg.repos.empty()) {
+                int total_channels = 0;
+                for (auto& r : cfg.repos) total_channels += r.channels.size();
+                std::cout << "                       repos: " << cfg.repos.size()
+                          << " (" << total_channels << " channels)\n";
+            }
             return 0;
         } catch (const std::runtime_error& e) {
             handle_parse_error(file_arg, source, e);
@@ -414,9 +446,17 @@ int main(int argc, char* argv[]) {
                     return 1;
                 }
                 return 0;
-            } catch (const std::runtime_error& e) {
-                handle_parse_error(file_arg, source, e);
-                return 1;
+            } catch (const std::runtime_error&) {
+                try {
+                    auto idx = dsl::parse_index(source);
+                    std::cout << file_arg << ": valid index ("
+                              << idx.name << ", "
+                              << idx.packages.size() << " packages)\n";
+                    return 0;
+                } catch (const std::runtime_error& e) {
+                    handle_parse_error(file_arg, source, e);
+                    return 1;
+                }
             }
         }
     }
@@ -472,9 +512,15 @@ int main(int argc, char* argv[]) {
                 auto cfg = dsl::parse_system_config(source);
                 tools::format_config(std::cout, cfg);
                 return 0;
-            } catch (const std::runtime_error& e) {
-                handle_parse_error(file_arg, source, e);
-                return 1;
+            } catch (const std::runtime_error&) {
+                try {
+                    auto idx = dsl::parse_index(source);
+                    tools::format_index(std::cout, idx);
+                    return 0;
+                } catch (const std::runtime_error& e) {
+                    handle_parse_error(file_arg, source, e);
+                    return 1;
+                }
             }
         }
     }