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.test.ts

@@ -167,6 +167,19 @@ describe("sanitizeUserSettings", () => {
 		assert.equal(notObject?.agent?.search, undefined);
 	});
 
+	it("graphTools subsection: boolean survives; bad shapes drop (default ON resumes) — wave 7, 2026-09-23", () => {
+		const on = sanitizeUserSettings({ version: 1, agent: { graphTools: { enabled: true } } });
+		assert.deepEqual(on?.agent?.graphTools, { enabled: true });
+		const off = sanitizeUserSettings({ version: 1, agent: { graphTools: { enabled: false } } });
+		assert.deepEqual(off?.agent?.graphTools, { enabled: false });
+		const bare = sanitizeUserSettings({ version: 1, agent: { graphTools: {} } });
+		assert.deepEqual(bare?.agent?.graphTools, {});
+		const badShape = sanitizeUserSettings({ version: 1, agent: { graphTools: "on" } });
+		assert.equal(badShape?.agent?.graphTools, undefined);
+		const badEnabled = sanitizeUserSettings({ version: 1, agent: { graphTools: { enabled: "yes" } } });
+		assert.equal(badEnabled?.agent?.graphTools, undefined);
+	});
+
 	it("partial maps: booleans survive, non-boolean entries drop individually", () => {
 		const out = sanitizeUserSettings({ agent: { tools: { quiz: false, demo: true, visual: "x" } } });
 		assert.ok(out);