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.

tests/web-search.test.ts

@@ -108,6 +108,7 @@ function fakeJobStore(): JobStore {
 		},
 		listJobs: () => [...jobs.values()],
 		findJobByToolCallId: () => null,
+		deleteJobsForChat: () => 0, // the drop tests never delete
 	};
 }