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

AGENTS.md

@@ -171,6 +171,20 @@ formatting is idempotent for the formatter.
   `NoSuchFileException: .../in-progress-results-generic.bin`, and no XML is
   written. It is infrastructure, not a test failure — `./jaitest` retries once
   automatically. Do not go debugging the test that "failed".
+- **A poisoned build cache can make tests silently vanish.** Symptom:
+  `BUILD SUCCESSFUL`, `:compileTestKotlin FROM-CACHE`, `:test NO-SOURCE`, and
+  `build/classes/kotlin/test` is empty. The cache stored an empty output
+  directory from a build whose outputs were deleted underneath it, and the entry
+  is keyed by input hash so it never gets replaced. This is the exact failure
+  mode `./jaitest`'s "total tests = 0 is a failure" rule exists to catch.
+  Recover with:
+
+  ```bash
+  ./jaigradle --stop
+  rm -rf build .gradle ~/.gradle/caches/build-cache-1
+  ./jaitest
+  ```
+
 - **`timeout` does not exist on this macOS box.** Do not use it in scripts.
 - **Noisy test stderr.** Fixture runs log Vue/JS `PluginException`s from bundled
   plugins. Tests pass regardless, but real failures can be buried — check the