feat(extraction): kill the dead LLM cluster-namer — session names own the labels now

Wave 7 chunk A (950ddfd) replaced semantic clustering with birth-session
clusters labeled by session names — the LLM namer became dead code, but
extraction still called it, minting naming runs nobody reads on every
merge. Real model spend wasted per extraction.

Removed (agent band):
- src/agent/subagents/cluster-namer.ts — the naming sub-agent, deleted
  whole (refreshClusterLabels, clusterNamingSettled, the naming loop).
- extraction.ts: both post-merge garnish blocks (chat + notes paths),
  the labeler test seam on ExtractionModels, and the import. The labeler
  role's only consumer is gone; the 'labeler' entry in
  BACKGROUND_MODEL_ROLES/ROLE_SLOT (core band) is now consumer-less —
  flagged to Platform, not crossed.
- scripts/import-notes.ts: the settle-before-exit line + the named-
  clusters count in the summary.
- tests: the cluster-naming describe (naming + failure-non-fatal tests)
  and the dead CLUSTER_* fixtures; the labeler-less BYOK test repurposed
  as the kill pin — a merge mints NO naming runs and NO label records.

NOT touched (flagged, outside the agent band): matchClusterLabelings
(core/clusters.ts), clusterContentId (core/graph.ts — now consumer-
less), the appendClusterLabel/readClusterLabels port seams + store
impls + ClusterLabeling type (store/core band), tests/clusters.test.ts's
matchClusterLabelings describe. Cost-summary's naming stage untouched
per Platform's ruling (historical spend display reads readNamingRuns,
which stays live).

Gates: typecheck, 921/921, lint, build.

scripts/import-notes.ts

@@ -16,7 +16,6 @@ import * as fs from "node:fs";
 import * as path from "node:path";
 import { createExtractionPipeline } from "../src/agent/subagents/extraction.ts";
 import { embedNewNodes } from "../src/server/embeddings.ts";
-import { clusterNamingSettled } from "../src/agent/subagents/cluster-namer.ts";
 import { getGraph } from "../src/core/graph.ts";
 import { stores } from "../src/server/stores.ts";
 
@@ -54,16 +53,10 @@ async function main() {
 	});
 	await Promise.all(workers);
 
-	// The post-merge cluster naming is fire-and-forget inside the pipeline;
-	// settle it before the process exits so labels actually land.
-	await clusterNamingSettled();
-
 	const graph = getGraph(stores.graphStore);
-	const labels = stores.graphStore.readClusterLabels().length;
 	console.log(
 		`[import-notes] done: +${totals.nodes} nodes, +${totals.edges} edges, +${totals.events} events, ${totals.failed} failed — ` +
-			`graph now ${graph.nodes.length} nodes, ${graph.edges.length} edges, ${graph.events.length} events, ` +
-			`${labels} named clusters`,
+			`graph now ${graph.nodes.length} nodes, ${graph.edges.length} edges, ${graph.events.length} events`,
 	);
 }