docs(mem0-plugin): comprehensive TESTING.md covering all Tiers 1-8

Rewrote TESTING.md with behavioral tests for every tier item:
- v3 endpoint verification for all 5 write scripts + search
- app_id scoping and branch tagging checks
- Skill content validation (confidence, citations, recency, infer=False)
- Robustness checks (loop guard, set flags, fallback stubs)
- 28-item checklist covering all 34 roadmap items
- Fixed stale comment in on_stop.sh

mem0-plugin/scripts/on_stop.sh

@@ -36,7 +36,7 @@ if [ -n "$REPORT" ]; then
   echo ""
 fi
 
-# Append to persistent session log (guarded — on_stop.sh uses set -euo pipefail)
+# Append to persistent session log
 if [ -n "$REPORT" ]; then
   mkdir -p "$HOME/.mem0" 2>/dev/null || true
   echo "$(date -u +%Y-%m-%dT%H:%M:%SZ) | $REPORT" >> "$HOME/.mem0/session-log.md" 2>/dev/null || true