feat: add kappa upgrade subcommand

Compares installed packages against cached indexes to find newer
versions. kappa upgrade shows available upgrades and updates the
system config with new version constraints. --dry-run shows what
would change without applying.

src/main.cpp

@@ -58,6 +58,7 @@ Subcommands:
                   --features "key=on,key2=off"   Set feature flags
                   --config   "key=val,key2=val2" Set config values
   remove       <pkg>      Remove a package from system config
+  upgrade                 Check indexes for newer versions of installed packages
 
 Options:
   -h, --help              Show this help message
@@ -226,7 +227,8 @@ int main(int argc, char* argv[]) {
                            || (subcommand == "index")
                            || (subcommand == "search")
                            || (subcommand == "add")
-                           || (subcommand == "remove");
+                           || (subcommand == "remove")
+                           || (subcommand == "upgrade");
 
     if (!valid_subcommand) {
         std::cerr << "error: unknown subcommand '" << subcommand << "'\n\n";
@@ -540,6 +542,109 @@ int main(int argc, char* argv[]) {
         }
     }
 
+    if (subcommand == "upgrade") {
+        auto installed = install::read_installed();
+        if (installed.empty()) {
+            std::cout << "no packages installed\n";
+            return 0;
+        }
+
+        // Build a map of latest versions from cached indexes
+        std::unordered_map<std::string, std::string> latest;
+        auto idx_dir = paths::cache_dir() / "indexes";
+        std::error_code ec;
+        if (std::filesystem::exists(idx_dir)) {
+            for (auto& entry : std::filesystem::directory_iterator(idx_dir, ec)) {
+                if (ec) break;
+                if (entry.path().extension() != ".kap") continue;
+                std::ifstream in(entry.path());
+                if (!in) continue;
+                std::ostringstream buf;
+                buf << in.rdbuf();
+                try {
+                    auto idx = dsl::parse_index(buf.str());
+                    for (auto& pkg : idx.packages) {
+                        auto it = latest.find(pkg.name);
+                        if (it == latest.end() || pkg.version > it->second) {
+                            latest[pkg.name] = pkg.version;
+                        }
+                    }
+                } catch (...) { continue; }
+            }
+        }
+
+        if (latest.empty()) {
+            std::cout << "no cached indexes — run kappa fetch-package <name> first\n";
+            return 0;
+        }
+
+        std::vector<std::pair<std::string, std::string>> upgrades;
+        for (auto& e : installed) {
+            auto it = latest.find(e.name);
+            if (it != latest.end() && it->second > e.version) {
+                upgrades.emplace_back(e.name, it->second);
+            }
+        }
+
+        if (upgrades.empty()) {
+            std::cout << "all " << installed.size() << " packages up to date\n";
+            return 0;
+        }
+
+        std::cout << upgrades.size() << " upgrade(s) available:\n";
+        for (auto& [name, ver] : upgrades) {
+            // Find old version
+            std::string old_ver;
+            for (auto& e : installed) {
+                if (e.name == name) { old_ver = e.version; break; }
+            }
+            std::cout << "  " << name << "  " << old_ver << " → " << ver << "\n";
+        }
+
+        if (dry_run) {
+            std::cout << "run kappa add <pkg> <version> for each to apply\n";
+            return 0;
+        }
+
+        // Apply: update system config with new versions
+        auto config_path = std::filesystem::path(paths::system_dir()) / "config.kap";
+        dsl::SystemConfig cfg;
+        if (std::filesystem::exists(config_path)) {
+            auto src = read_file(config_path.c_str());
+            try {
+                cfg = dsl::parse_system_config(src);
+            } catch (...) { cfg = {}; }
+        }
+
+        for (auto& [name, ver] : upgrades) {
+            bool found = false;
+            for (auto& p : cfg.packages) {
+                if (p.name == name) {
+                    p.version = ver;
+                    found = true;
+                    break;
+                }
+            }
+            if (!found) {
+                dsl::PackageRef pref;
+                pref.name = name;
+                pref.version = ver;
+                cfg.packages.push_back(std::move(pref));
+            }
+        }
+
+        std::filesystem::create_directories(paths::system_dir(), ec);
+        std::ofstream out(config_path);
+        if (!out) {
+            std::cerr << "error: cannot write config\n";
+            return 1;
+        }
+        tools::format_config(out, cfg);
+
+        std::cout << "config updated — run kappa rebuild to apply\n";
+        return 0;
+    }
+
     if (subcommand == "fetch-package") {
         if (file_arg == nullptr) {
             std::cerr << "error: no package name specified\n";