fix(mem0-plugin): sanitize marker path, add pre-compact REST backup, fix cross-tool gaps

- Sanitize project_id in onboard marker path (tr '/:' '--') to handle
  slashes from env var overrides
- Fire on_pre_compact.py REST backup from PreCompact hook (was only
  called from Stop hooks — crash before Stop = data loss)
- Source _identity.sh in on_stop_codex.sh and on_stop_cursor.sh so
  MEM0_PROJECT_ID + API key fallback are available
- Note dream --schedule is Claude Code-only (Codex/Cursor lack schedule)

mem0-plugin/scripts/on_session_start.sh

@@ -79,7 +79,8 @@ echo ""
 
 if [ "$SOURCE" = "startup" ]; then
   # First-run detection: auto-trigger onboarding for new projects
-  _ONBOARD_MARKER="$HOME/.mem0/.onboarded_${MEM0_PROJECT_ID}"
+  _SAFE_PID=$(printf '%s' "$MEM0_PROJECT_ID" | tr '/:' '--')
+  _ONBOARD_MARKER="$HOME/.mem0/.onboarded_${_SAFE_PID}"
   if [ ! -f "$_ONBOARD_MARKER" ]; then
     cat <<'EOF'
 ## Mem0 First Run — Automatic Onboarding