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_stop_cursor.sh

@@ -14,6 +14,8 @@ if [ -n "${MEM0_DEBUG:-}" ]; then
 fi
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck source=_identity.sh
+. "$SCRIPT_DIR/_identity.sh"
 
 INPUT=$(cat)