Semantic topic clusters: local embeddings replace Louvain

Louvain only ever found the reasoning chains (the graph is disconnected
by nature — every connected component became its own cluster) and
edge-orphans could never join anything. Clusters are now RELATED TOPICS:
propositions grouped by meaning.

- Embedder (src/server/embeddings.ts): in-process transformers.js v4
  running EmbeddingGemma-300M q8 (MTEB 68.4, ~300MB one-time download),
  MRL-truncated to 512d, "title: none | text:" prefix on 100% of inputs,
  L2-normalized so cosine = dot. Model/dim mismatch voids the ledger
  (full re-embed). Injected at the composition root — the agent layer
  never imports the server layer (architecture law).
- Ledger (data/graph/embeddings.json): model + dim + vectors + the
  persisted cluster partition. Reclustering runs at the END of every
  embedding batch (post-merge hook, both front doors) — never per read
  (the agglomerative pass is ~1.5s at 338 nodes).
- Clustering (src/core/semantic-clusters.ts): hand-rolled average-linkage
  agglomerative over cosine (npm HDBSCAN is an abandoned alpha). Stragglers
  stay singletons by default; attachStragglers is the all-in toggle.
  Threshold 0.60 swept on the real graph: 26 topics, 74/90 edge-orphans
  land in real topics, 10 stragglers.
- Consumers (snapshot, settle, namer, replay) read the persisted
  partition — one cluster concept everywhere; core-survival label
  matching unchanged (20/26 topics kept labels).
- Louvain retired: detectCommunities + graphology* deps deleted
  (graphology-louvain had leaked into the client bundle since the
  clusters wave); scripts/bench-graph.ts (dead FA2 bench) removed.
- scripts/embed-graph.ts: backfill/cold-start (338 nodes in 150s
  including the one-time model download).

scripts/import-notes.ts

@@ -15,6 +15,7 @@
 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";
@@ -28,7 +29,7 @@ async function main() {
 		.sort();
 	console.log(`[import-notes] ${files.length} note files from ${dir} (concurrency ${limit})`);
 
-	const pipeline = createExtractionPipeline(stores);
+	const pipeline = createExtractionPipeline({ ...stores, embedder: embedNewNodes });
 	const queue = [...files];
 	const totals = { nodes: 0, edges: 0, events: 0, failed: 0 };
 	const workers = Array.from({ length: Math.min(limit, files.length) }, async () => {