Session stop re-runs graph extraction over the full observation set even when nothing changed

[yulin0629]

Summary

The event::session::stopped handler sends the session's entire compressed observation set to mem::graph-extract on every session stop, with no change detection. Sessions that stop repeatedly without new activity (heartbeat-style loops) re-extract the identical set every time.

Root cause

src/triggers/events.ts (session-stopped handler):

if (isGraphExtractionEnabled()) {
  const observations = await kv.list<CompressedObservation>(
    KV.observations(data.sessionId),
  );
  const compressed = observations.filter((o) => o.title);
  if (compressed.length > 0) {
    sdk.trigger({
      function_id: "mem::graph-extract",
      payload: { observations: compressed },   // full set, every stop
      action: TriggerAction.Void(),
    });
  }
}

There is no fingerprint/dedup — every stop re-processes the full history even when zero observations were added since the last extraction.

Observed impact (self-hosted, v0.9.27)

A session capped at 500 observations that stopped every 30 minutes produced a ~213 KB (~50K input tokens) graph-extraction LLM request per cycle, around the clock, with the input identical each time (extraction yields mostly already-known nodes/edges).

This is the same shape as the double/redundant summarize issue (#1062) — full re-processing on session stop with no change detection — just in a different function.

Fix

Included in #1061 (shares the computeInputFingerprint helper introduced there): fingerprint the compressed set, store it per session (KV.graphExtractState), and trigger mem::graph-extract only when the fingerprint changed.