Birth-session clustering + the prober graph-tools data seam (wave 7 chunk A)

Embeddings are dying (chunk B removes the machinery); the clusters they
used to carry become the session that created them — derived on read.

- New src/core/session-clusters.ts: birthSessionPartition groups merged
  nodes by their FIRST SURVIVING provenance point's chatId (owner-locked
  2026-09-23). Deletion is self-healing by construction: stripped-birth
  survivors re-attribute to their earliest remaining contributor, and
  empty-provenance nodes stay unclustered. Notes imports are birth-sessions
  too (owner ruling) — their label is the file path.
- graphSnapshot: clusters = the birth partition filtered >= MIN_CLUSTER_SIZE
  (the hull presentation rule), id = session id, label = session name (chat
  title / file path; a deleted record renders unlabeled). The persisted
  cluster-labelings join dies; no LLM naming pass needed anymore.
- settleGraphLayout derives the same partition from readNodes — drops its
  readEmbeddings dep. One cluster concept everywhere.
- Prober data seam (core/graph.ts, pure ports-driven like the snapshot):
  graphSessionIndex (every birth session: id, name, chat|file kind,
  createdAt/updatedAt, birth-proposition count, recency-ordered) and
  graphSessionPropositions (a session's birth propositions with the
  extractor's latest understanding level and THAT event's own timestamp —
  owner ruling: quiz outcomes are ledger evidence, they never move the
  evaluation date). Teacher's tool handlers consume these in-process.
- New agent.graphTools subsection { enabled?: boolean } (owner ruling:
  the toggle gates prober tool REGISTRATION — agent capability, agent
  section, its own subsection, NOT the tool-toggles map; absent = ON).
  Lenient sanitizer, strict settings-body parser, passthrough resolution,
  and graphToolsEnabled(effective) = graph.enabled && toggle for Teacher's
  registry wiring.

Gates: tsc clean, eslint clean, 923/923 tests, next build compiles.
Live-verified on the worktree dev data: 10 nodes → 2 clusters labeled
with their chat titles; session index + propositions carry levels and
their own timestamps.

tests/settings-resolution.test.ts

@@ -12,6 +12,7 @@ import { describe, it } from "node:test";
 import {
 	extractionEnabled,
 	graphContextEnabled,
+	graphToolsEnabled,
 	resolveConversationSpec,
 	resolveModelSpec,
 	resolveRoleSpec,
@@ -243,4 +244,20 @@ describe("the graph channel gates (the wave's seam honoring)", () => {
 		assert.equal(extractionEnabled(off.graph), false);
 		assert.equal(graphContextEnabled(off.graph), false);
 	});
+
+	it("PROBER GRAPH TOOLS gate (wave 7, owner ruling 2026-09-23): the agent toggle AND the graph master switch compose", () => {
+		const graph = { enabled: true, embedding: true, clustering: true, labeling: true, embeddingModel: null };
+		assert.equal(graphToolsEnabled({ graph, agent: { graphTools: {} } }), true, "absent subsection = ON");
+		assert.equal(graphToolsEnabled({ graph, agent: { graphTools: { enabled: true } } }), true);
+		assert.equal(graphToolsEnabled({ graph, agent: { graphTools: { enabled: false } } }), false);
+		const dead = { ...graph, enabled: false };
+		assert.equal(graphToolsEnabled({ graph: dead, agent: { graphTools: { enabled: true } } }), false, "graph off kills the tools regardless of the toggle");
+	});
+
+	it("graphTools rides resolveSettings: absent record → the empty subsection (ON); a record's delta passes through", () => {
+		const eff = resolveSettings(null);
+		assert.deepEqual(eff.agent.graphTools, {});
+		const off = resolveSettings({ version: 1, agent: { graphTools: { enabled: false } } });
+		assert.deepEqual(off.agent.graphTools, { enabled: false });
+	});
 });