Build an installable plugin ZIP: real README/CHANGELOG, cache-poisoning recovery

- README documents install-from-disk and what actually works today
- jaitest prints the recovery steps when a poisoned build cache makes
  :test report NO-SOURCE, and AGENTS.md records the failure mode

jaitest

@@ -38,6 +38,14 @@ echo
 echo "=== JUnit XML ==="
 if [ ${#xml[@]} -eq 0 ]; then
     echo "NO TEST XML PRODUCED — tests did not run."
+    echo
+    echo "If Gradle reported ':test NO-SOURCE' while the sources clearly exist, the"
+    echo "build cache holds a poisoned entry with empty compileTestKotlin outputs"
+    echo "(a concurrent build had its outputs deleted mid-run). Recover with:"
+    echo
+    echo "    ./jaigradle --stop"
+    echo "    rm -rf build .gradle ~/.gradle/caches/build-cache-1"
+    echo "    ./jaitest"
     exit 1
 fi