chore: merge upstream v12.3.1 + keep local fixes

Upstream brings:
- 12.2.1: Break infinite summary-retry loop (#2072)
- 12.2.2: Subagent observation labeling + schema migration (#2073)
- 12.2.3: Silence parser warning on normal observation responses (#2074)
- 12.3.0: Docker harness + SWE-bench eval harness (#2076)
- 12.3.1: Error handling anti-pattern cleanup across 91 files (#2078)

Local fixes preserved through merge:
- env-sanitizer PATH extension for claude CLI lookup
- SessionStore stale session reset (mac sleep / 4h wall-clock)

Built artifacts rebuilt from merged sources; fixes verified present
in worker-service.cjs.

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

plugin/.claude-plugin/plugin.json

@@ -1,6 +1,6 @@
 {
   "name": "claude-mem",
-  "version": "12.2.0",
+  "version": "12.3.1",
   "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": "12.2.0",
+  "version": "12.3.1",
   "private": true,
   "description": "Runtime dependencies for claude-mem bundled hooks",
   "type": "module",