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/jobs.ts

@@ -79,10 +79,28 @@ function findJobByToolCallId(chatId: string, toolCallId: string): JobRecord | nu
 	return listJobs(chatId).find((j) => j.toolCallId === toolCallId) ?? null;
 }
 
+/** Delete every job record file for a chat (session-deletion wave 6) —
+ *  running or finished, they are the chat's records and die with it.
+ *  Returns how many files were removed. The deletion route refuses while
+ *  jobs are still running, so every record here is terminal. */
+function deleteJobsForChat(chatId: string): number {
+	let removed = 0;
+	for (const job of listJobs(chatId)) {
+		try {
+			fs.unlinkSync(jobFile(job.id));
+			removed++;
+		} catch {
+			/* already gone — the count is best-effort truth */
+		}
+	}
+	return removed;
+}
+
 export const jobStore: JobStore = {
 	createJob,
 	getJob,
 	updateJob,
 	listJobs,
 	findJobByToolCallId,
+	deleteJobsForChat,
 };