docs: chains-primary reality in architecture.md + README; two stale comments fixed

- architecture.md section 6 rewritten: stored truth = propositions + chains
  (chains.jsonl); deriveView as the lossy image (implication nodes +
  untyped arrows, rebuilt per read); the bijection framing (independent
  arguments for one conclusion stay distinct — the corruption the edge
  store invited); new deletion-semantics paragraph (proposition death
  cascades to every referencing chain; arg_ death = one argument);
  supports retirement stated plainly; auditor's propositions-only scope;
  clusters stated as birth-session partition truth (no Louvain claim)
- section 7 practice chain model: stored-chains inclusion law replaces
  follows-edge grouping; chainsToGraph bijection claim deleted
- README: graph clauses (stored chains, derived view, chain dedupe),
  practice clause, ledger inventory, and two long-stale line-5 claims
  (ledger events, semantic top-K) corrected
- contracts/graph.ts GraphSnapshot comment: Louvain claim replaced with
  birth-session truth; extraction.ts comments no longer narrate the
  dead expandFindings/chainsToEdges seam

src/core/contracts/graph.ts

@@ -322,8 +322,11 @@ export interface GraphSnapshot {
 	fileSources: Record<string, string>;
 	/** nodeId → chatIds that touched the node (click-through provenance). */
 	nodeSessions: Record<string, string[]>;
-	/** Structural communities (Louvain over follows+supports), size ≥ 3,
-	 * joined with persisted LLM labelings. Purely structural — never
+	/** Birth-session communities (core/session-clusters.ts: each
+	 * proposition joins the cluster of the session that birthed it —
+	 * session provenance, no Louvain, no embeddings), MIN_CLUSTER_SIZE
+	 * ≥ 3 as the hull presentation rule. Propositions only — implication
+	 * nodes are never partitioned. Purely provenance-derived — never
 	 * depends on layout. */
 	clusters: Cluster[];
 	/** Resting layout positions per node id, when a positions ledger