Clusters phases B+C: labeling records, the naming call, snapshot payload

Core-survival labelings (vision section 1, DECIDED 2026-09-13):
ClusterLabeling records in a new append-only ledger
(data/graph/cluster-labels.jsonl), immutable, id = clu_ + sha256 of the
sorted core so re-naming races and re-runs dedupe at the store seam.
matchClusterLabelings (core/clusters.ts, pure): a community keeps a
record's label while at least half its core survives in it; both sides
single-successor (a split keeps one name; greedy by overlap, ties to
the newer record).

The namer (src/agent/subagents/cluster-namer.ts): ONE singular cheap LLM
call (new 'labeler' role, glm-flash/low) names every new/changed
community at once, given member names + statements (capped at 12
members per cluster). refreshClusterLabels runs POST-EXTRACTION
(user-decided) from both front doors, fire-and-forget — naming failure
never fails the run; refreshes serialize behind a chain so parallel
imports never mint duplicate labels. import-notes.ts awaits
clusterNamingSettled before exit.

Payload: GraphSnapshot.clusters (>= MIN_CLUSTER_SIZE 3) joins detection
with labelings at read time — claimed clusters carry the record's label
and id; unlabeled ones carry a member-hash id and null label. Detection
is deterministic so no memo in v1 (ms at current scale; revisit at
vault scale).

Tests (372/372): matcher half-rule/split/merge/greedy/tie cases; naming
end-to-end through importNotes (one call, one record, claim on
re-refresh), naming failure non-fatal; snapshot derivation with
labeling join; 11 test pipelines now pin an explicit labeler seam.

src/core/ports.ts

@@ -25,6 +25,7 @@ import type {
 	SessionCheckpoint,
 	TurnUsage,
 	NotesSource,
+	ClusterLabeling,
 } from "./contracts.ts";
 
 // ─── ChatStore — the session record's home (data/chats) ─────────────────────
@@ -82,6 +83,11 @@ export interface GraphStore {
 	appendCheckpoint(checkpoint: SessionCheckpoint): void;
 	readCheckpoints(): SessionCheckpoint[];
 
+	/** Append one cluster labeling — idempotent by id (the id hashes the
+	 *  sorted core, so identical member sets dedupe). */
+	appendClusterLabel(labeling: ClusterLabeling): void;
+	readClusterLabels(): ClusterLabeling[];
+
 	/** Upsert one imported-notes source record (config.SOURCES_FILE — a
 	 *  JSON map, not a ledger; an upsert keeps the existing usage log). */
 	upsertSource(source: NotesSource): void;