deploy: domain-collision pre-check with clear error

action.yml

@@ -138,6 +138,22 @@ runs:
           fi
         fi
 
+        # ---- domain collision pre-check: clear error instead of a raw caddy
+        #      "ambiguous site definition" when another deploy already serves this domain ----
+        for dom in "$DOMAIN" ${ALT:+"$ALT"}; do
+          for frag in /srv/platform/caddy/sites/*.caddy; do
+            [ -f "$frag" ] || continue
+            base=$(basename "$frag")
+            { [ "$base" = "${SLUG}.caddy" ] || [ "$base" = "_platform.caddy" ]; } && continue
+            for a in $(sed -n '1,/{/p' "$frag" | tr -d '{' | tr ',' ' '); do
+              if [ "$a" = "$dom" ]; then
+                echo "::error::domain '$dom' is already served by another deploy ($base). Change DOMAIN in your .env, or ask the admin to free it (remove $base)."
+                exit 1
+              fi
+            done
+          done
+        done
+
         case "$DT" in
           static)
             # build IN-PLACE — the job runs on platform-ci-base which has bun, so no