Multiloader scaffold: Architectury common+fabric+forge+neoforge (1.21.1)

Convert the single-loader NeoForge project into an Architectury multiloader:
- root/settings/subproject build.gradle, Gradle pinned to 8.10.2 (Loom needs 8.x)
- common: generic SkinsgCommands<S> + SkinsgClient bootstrap
- per-loader entrypoints (Fabric ClientModInitializer, Forge/NeoForge @Mod) + metadata
- migrate-to-multiloader.sh moves the loader/version-agnostic core into common/
- MULTILOADER.md: build/toolchain/proxy + 1.20.1 switch incl. rewritten cape mixin

main (working NeoForge 1.21.1) untouched.

settings.gradle

@@ -1,10 +1,16 @@
 pluginManagement {
     repositories {
+        maven { url "https://maven.fabricmc.net/" }
+        maven { url "https://maven.architectury.dev/" }
+        maven { url "https://maven.minecraftforge.net/" }
+        maven { url "https://maven.neoforged.net/releases/" }
         gradlePluginPortal()
-        maven { url = 'https://maven.neoforged.net/releases' }
     }
 }
 
-plugins {
-    id 'org.gradle.toolchains.foojay-resolver-convention' version '1.0.0'
-}
+rootProject.name = "skinsg"
+
+include("common")
+include("fabric")
+include("forge")
+include("neoforge")