training: per-node scoring with graph weight updates

Memory scoring now uses the graph as source of truth:
- last_scored timestamp on each node (new capnp field @22)
- Nodes scored when older than scoring_interval_secs (default 1hr)
- Oldest-scored-first ordering
- Window: scoring_response_window assistant responses (default 100)
- First-quarter memories scored even without full window
- Per-response normalization (raw divergence / response count)
- Asymmetric weight update: alpha=0.5 up, alpha=0.1 down
  (responds fast to importance, decays slowly — memories stay
  surfaced even if only useful 1/4 of the time)

Graph writes disabled pending normalization calibration.

Also: configurable scoring_interval_secs and scoring_response_window.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

schema/memory.capnp

@@ -42,6 +42,9 @@ struct ContentNode {
   # Freeform provenance string: "extractor:write", "rename:tombstone", etc.
   provenance @21 :Text;
 
+  # Memory importance scoring
+  lastScored @22 :Int64;     # unix epoch seconds, 0 = never scored
+
 }
 
 enum NodeType {