chains-primary: stored chains replace typed edges; edges become a derived view

Owner rulings 2026-09-30: reasoning chains are the stored truth; the graph
canvas renders a lossy derived image. No migration, no backcompat — old
graph data was wiped.

- contracts: GraphChain (arg_-minted id, premises, conclusion) replaces
  typed edges; GraphEdge survives only as the untyped derived arrow;
  GraphSnapshot carries stored chains; supports edges retired entirely
- store: chains.jsonl ledger (edges.jsonl purged at load alongside legacy
  events.jsonl); readChains/appendChain seam; merge dedupes by
  (sorted premises, conclusion), re-assertions append under the same arg_ id
- core: deriveView mints implication nodes (name ⟹, statement via
  formatChainLine) plus premise→imp→conclusion arrows; clusters stay
  proposition-only (birthSessionPartition); chainsForSelection now groups
  STORED chains (chain rides iff every member selected) — independent
  arguments for one conclusion never union; chainsToGraph/chainsToEdges die
- extraction: extractor submits chains natively (expandFindings deleted);
  auditor receives propositions only; supports fan-out gone
- deletion: full death of a proposition kills every referencing chain;
  arg_-prefixed node deletes kill that chain; chainsRemoved stats
- UI: every arrow renders the ⟹ double shaft; supports toggle + thickness
  slider and detail-card edge sections deleted (card keeps name +
  proposition); chain table + practice pipeline read stored chains
- tests: 1188 green across the new seams

src/agent/subagents/practice-pipeline.ts

@@ -45,6 +45,7 @@
 import type { GraphStore, PracticeStore } from "../../core/ports.ts";
 import type { Problem, ProblemSet, ReasoningChain } from "../../core/contracts.ts";
 import { chainsForSelection } from "../../core/practice.ts";
+import { getGraph } from "../../core/graph.ts";
 import {
 	createPracticeAgents,
 	type PracticeModels,
@@ -444,10 +445,10 @@ export function createPracticePipeline(deps: PracticePipelineDeps) {
 		const wanted = new Set(nodeIds);
 		const selected = deps.graphStore.readNodes().filter((n) => wanted.has(n.id));
 		const selectedIds = new Set(selected.map((n) => n.id));
-		const edges = deps
-			.graphStore.readEdges()
-			.filter((e) => e.type === "follows" && selectedIds.has(e.from) && selectedIds.has(e.to));
-		const chains = chainsForSelection(selected, edges);
+		// The STORED chains (chains-primary, owner 2026-09-30): chainsForSelection
+		// keeps a chain iff every member is selected — the same inclusion law as
+		// the pre-chains edge grouping; never the derived fan-out.
+		const chains = chainsForSelection(selected, getGraph(deps.graphStore).chains);
 		if (chains.length === 0) throw new EmptySelectionError();
 		const propositions: PropositionSnapshot = {};
 		for (const n of selected) propositions[n.id] = { name: n.name, statement: n.statement };