feat: add investigation report on stuck observations in processing state

- Documented six critical gaps in the message lifecycle leading to stuck observations.
- Detailed analysis of message lifecycle architecture, including status states and normal flow.
- Identified critical stuck points with code paths and required fixes.
- Proposed recovery mechanisms and recommendations for critical fixes, robustness improvements, and observability enhancements.
- Included file references for database, processing, HTTP, worker layers, hooks, and constants.

plugin/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem-plugin",
-  "version": "8.5.3",
+  "version": "8.5.4",
   "private": true,
   "description": "Runtime dependencies for claude-mem bundled hooks",
   "type": "module",