feat(systemd): Dashboard-Service, brain_rules, 18 Engramme bewertet, Cron persistent

Neu:
- systemd: secondbrain-dashboard.service (Port 8501, autostart)
- cron_rules.py: Auto-Confirm ab 3x, Archiv nach 30d
- cron_tasks/: heartbeat + backup + brain_rules (persistent)
- openclaw_cron_wrapper.py: subprocess-Isolation (kein SessionTakeover)
- chat_autosave.py: Auto-Save von Chat + Kontext-Anreicherung

Daten:
- 18 unbestätigte Engramme bewertet:
  - 14x CONFIRMED (Fakten/Definitionen korrekt)
  - 3x ARCHIVIERT (historisch, nicht aktuell)
  - 1x CONFIRMED (Regel 73624013)
- 0 offene unbestätigte

Closes Gitea-Issue: #9

cron_tasks/heartbeat_secondbrain.py

@@ -0,0 +1,47 @@
+#!/usr/bin/env python3
+"""
+Heartbeat-Task für Second Brain - isoliert, persistent.
+"""
+
+import json
+import os
+import sys
+from pathlib import Path
+from datetime import datetime, timezone
+
+BRAIN_DIR = Path("/root/.openclaw/workspace/second-brain")
+sys.path.insert(0, str(BRAIN_DIR))
+
+from src.engram import Engram, Grounding
+from src.store import EngramStore
+
+def main():
+    output_file = os.environ.get("CRON_OUTPUT_FILE", "/tmp/heartbeat_result.json")
+    brain_db = os.environ.get("BRAIN_DB", str(BRAIN_DIR / "data" / "brain.sqlite"))
+    store = EngramStore(brain_db)
+
+    egs = store.get_all(limit=50)
+    unconfirmed = [eg for eg in egs if not eg.correctness.confirmed and eg.compute_confidence() > 0.5][:5]
+    errors = store.search_tag("error", limit=5)
+
+    result = {
+        "timestamp": datetime.now(timezone.utc).isoformat(),
+        "total_engrams": len(egs),
+        "unconfirmed_count": len(unconfirmed),
+        "error_count": len(errors),
+        "has_action": bool(unconfirmed) or len(errors) >= 3,
+        "message": None,
+    }
+
+    if unconfirmed:
+        contents = "\n".join([f"  - {eg.content[:80]}" for eg in unconfirmed])
+        result["message"] = f"🧠 Unbestätigte Engramme:\n{contents}"
+    elif len(errors) >= 3:
+        result["message"] = f"⚠️ {len(errors)} Fehler-Engramme gespeichert."
+
+    Path(output_file).write_text(json.dumps(result, indent=2))
+    print(f"HEARTBEAT: {result['unconfirmed_count']} unconfirmed, {result['error_count']} errors")
+    return 0
+
+if __name__ == "__main__":
+    sys.exit(main())