refactor: clean COMMUNITY_SCRIPTS_URL pattern (#1449)

- ct/vm scripts: split to 2-line pattern (variable + source)
- func files: remove redundant inline defaults
- Only entry points keep the default (ct/*.sh, vm/*.sh, build.func,
  install.func, alpine-install.func)
- Downstream sources use $COMMUNITY_SCRIPTS_URL without default

misc/vm-core.func

@@ -190,7 +190,7 @@ silent() {
   if [[ $rc -ne 0 ]]; then
     # Source explain_exit_code if needed
     if ! declare -f explain_exit_code >/dev/null 2>&1; then
-      source <(curl -fsSL "${COMMUNITY_SCRIPTS_URL:-https://git.community-scripts.org/community-scripts/ProxmoxVED/raw/branch/main}/misc/error_handler.func") 2>/dev/null || true
+      source <(curl -fsSL "$COMMUNITY_SCRIPTS_URL/misc/error_handler.func") 2>/dev/null || true
     fi
 
     local explanation=""