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/tools/format.cpp

@@ -237,6 +237,34 @@ void format_config(std::ostream& os, const dsl::SystemConfig& cfg) {
         os << "]\n\n";
     }
 
+    if (!cfg.repos.empty()) {
+        os << "repos {\n";
+        for (auto& repo : cfg.repos) {
+            os << "    " << repo.name << " {\n";
+            os << "        url = \"" << repo.url << "\"\n";
+            if (!repo.channels.empty()) {
+                os << "        channels = [";
+                for (std::size_t i = 0; i < repo.channels.size(); ++i) {
+                    if (i > 0) { os << ", "; }
+                    os << '"' << repo.channels[i] << '"';
+                }
+                os << "]\n";
+            }
+            if (!repo.mirrors.empty()) {
+                os << "        mirrors = [\n";
+                for (auto& m : repo.mirrors) {
+                    os << "            \"" << m << "\",\n";
+                }
+                os << "        ]\n";
+            }
+            if (repo.priority != 50) {
+                os << "        priority = " << repo.priority << "\n";
+            }
+            os << "    }\n";
+        }
+        os << "}\n\n";
+    }
+
     if (!cfg.assertions.empty()) {
         os << "assert {\n";
         for (auto& a : cfg.assertions) {
@@ -345,4 +373,12 @@ void format_config(std::ostream& os, const dsl::SystemConfig& cfg) {
     }
 }
 
+void format_index(std::ostream& os, const dsl::IndexDef& idx) {
+    os << "index \"" << idx.name << "\" {\n";
+    for (auto& pkg : idx.packages) {
+        os << "    " << pkg.name << " { version = \"" << pkg.version << "\" }\n";
+    }
+    os << "}\n";
+}
+
 } // namespace kappa::tools