fix: mcpp auto-resolves xpkg from global xlings, remove all CI workarounds

When xlings installs a toolchain, the payload may land in xlings'
global data dir instead of mcpp's sandbox. package_fetcher now checks
~/.xlings/data/xpkgs/ as fallback and copies automatically.

Removed from CI/release:
- xlings install llvm (mcpp handles it)
- Pre-seed cp -r of LLVM xpkgs (mcpp handles it)

mcpp now works like a real user: just `xlings install mcpp` then `mcpp build`.

Author: Sunrisepeak

src/pm/package_fetcher.cppm

@@ -605,6 +605,28 @@ Fetcher::resolve_xpkg_path(std::string_view target,
     };
 
     auto resolve = [&]() -> std::expected<XpkgPayload, CallError> {
+        // xlings may install the package into its global home rather than
+        // the mcpp sandbox. If the expected path is missing, copy from the
+        // global xlings data directory.
+        if (!std::filesystem::exists(verdir)) {
+            auto xhome = std::getenv("HOME");
+#if defined(_WIN32)
+            if (!xhome) xhome = std::getenv("USERPROFILE");
+#endif
+            if (xhome) {
+                auto globalDir = std::filesystem::path(xhome)
+                    / ".xlings" / "data" / "xpkgs"
+                    / verdir.parent_path().filename()
+                    / verdir.filename();
+                std::error_code ec;
+                if (std::filesystem::exists(globalDir, ec)) {
+                    std::filesystem::create_directories(verdir.parent_path(), ec);
+                    std::filesystem::copy(globalDir, verdir,
+                        std::filesystem::copy_options::recursive
+                        | std::filesystem::copy_options::overwrite_existing, ec);
+                }
+            }
+        }
         if (!std::filesystem::exists(verdir)) {
             // xlings may have installed the package into its own global home
             // rather than the mcpp sandbox. Try to copy it from the global