chore: merge upstream v12.3.8 + keep local fixes

Upstream:
- 12.3.8: detect PID reuse in worker start-guard (#2082) — fixes
  docker container restart where new worker inherits the old PID
  and kill(pid, 0) falsely reports the old instance alive. Uses
  /proc/<pid>/stat starttime on Linux and `ps -p <pid> -o lstart=`
  on macOS/POSIX as an opaque process-start identity token.

Low impact for macOS Desktop users but worth carrying.

Local fixes preserved: env-sanitizer PATH extension, SessionStore
stale session reset. Both verified in built worker-service.cjs.
Worker restarted to v12.3.8.

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

plugin/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem",
-  "version": "12.3.7",
+  "version": "12.3.8",
   "description": "Persistent memory system for Claude Code - seamlessly preserve context across sessions",
   "author": {
     "name": "Alex Newman"

plugin/package.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem-plugin",
-  "version": "12.3.7",
+  "version": "12.3.8",
   "private": true,
   "description": "Runtime dependencies for claude-mem bundled hooks",
   "type": "module",