refactor(runtime): rename extras/ to runtime/, drop obsolete modrinth list

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

runtime/README.md

@@ -0,0 +1,11 @@
+# /runtime
+
+Runtime drop-ins mounted read-only into the Minecraft container at `/extras`.
+
+## Required files
+
+- `authlib-injector.jar` — download latest from
+  https://github.com/yushijinhun/authlib-injector/releases
+
+Mods are no longer listed here; the pack is served via packwiz
+(`PACKWIZ_URL`, see plan/04-packwiz.md).