feat(graph): two-mode session deletion — the first destructive ledger operation

DELETE /api/chats/[id]?mode=preserve-graph|delete-nodes (mode required):
- preserve-graph: only extraction bookkeeping dies (coverage rows,
  legacy checkpoints) — graph ledgers byte-untouched
- delete-nodes: the chat's ledger traces die. Node survival = surviving
  provenance point / mapping line (notes imports carry empty
  provenance) / surviving event. The creator's aliases + provenance are
  stripped from a surviving node's create line (owner ruling
  2026-09-23: name/statement/createdAt are the shared identity);
  orphan nodes, their edges, events, mappings, positions, embedding
  vectors and cluster members die; cluster labels stay (immutable)

Gates: 404 unknown, 409 turn_in_flight / jobs_running (with jobIds),
503 extraction_winding_down after a 10s abort-and-poll; the purge is
one sync block (ledgers → jobs → chat record last). rewriteLedgers is
atomic (tmp + rename). graphRevision (max mtime over graph data files)
rides the activity feed + the response as the change stamp.

Anti-resurrection: the chat pipeline re-checks chat liveness at each
merge — a run that slips past the wind-down aborts with nothing merged
or checkpointed; its partial spend dies with the chat.

src/store/chats.ts

@@ -129,6 +129,19 @@ function renameChat(id: string, title: string): ChatRecord {
 	return record;
 }
 
+/** Delete the session record file entirely (session-deletion wave 6,
+ *  owner 2026-09-22). False when the id is unknown — the route turns that
+ *  into a 404. The record's graph traces and jobs are the route's
+ *  choreography (core/graph-deletion.ts), not the store's. */
+function deleteChat(id: string): boolean {
+	try {
+		fs.unlinkSync(chatFile(id));
+		return true;
+	} catch {
+		return false; // never existed, already deleted, or unreadable — same answer
+	}
+}
+
 /** Persist the session plan tree (plan tools call this in execute). */
 function saveTree(id: string, tree: PlanTree): void {
 	const record = getChat(id);
@@ -147,6 +160,7 @@ export const chatStore: ChatStore = {
 	saveTree,
 	saveSettings,
 	renameChat,
+	deleteChat,
 	appendTurnUsage,
 	appendExtractionUsage,
 };