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/skills/mem0-onboard/SKILL.md

@@ -77,7 +77,8 @@ If the script reports an error, show the error message and suggest checking the
 Create a marker file so SessionStart won't re-trigger onboarding next session:
 
 ```bash
-mkdir -p ~/.mem0 && touch ~/.mem0/.onboarded_<active_project_id>
+_SAFE_PID=$(printf '%s' "<active_project_id>" | tr '/:' '--')
+mkdir -p ~/.mem0 && touch ~/.mem0/.onboarded_${_SAFE_PID}
 ```
 
 This is silent — no user-facing output needed.