release: v2.3.0 — first fully stable release

Bump version to 2.3.0 across package.json, openclaw.plugin.json.
Add comprehensive CHANGELOG documenting all bugs found and fixed
during the v2.x development cycle.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "memory-continuity",
-  "version": "2.2.0",
+  "version": "2.3.0",
   "description": "Zero-dependency memory continuity for OpenClaw — plain markdown, lifecycle hooks, no vector DB.",
   "main": "index.js",
   "type": "module",