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

CHANGELOG.md

@@ -1,5 +1,13 @@
 <!-- Keep a Changelog guide -> https://keepachangelog.com -->
 
-# Intellijai Changelog
+# Jai Plugin Changelog
 
 ## [Unreleased]
+
+### Added
+
+- `.jai` file type, icon, and language registration
+- Hand-written Jai lexer covering the full token set of the compiler's own lexer,
+  including nesting block comments and here-strings
+- Syntax highlighting and a colour settings page
+- Comment/uncomment support and brace matching, including `.{` and `.[`