feat(T2a): tool-toggle seam + phase-based question principles

Part 1 — tool availability (the settings seam, machinery generic):
- core/pending.ts: TOGGLEABLE_TOOLS (today: research, demo, visual —
  the model-facing tool names), EnabledTools = the JSON-native seam
  shape Partial<Record<string, boolean>> (absent key = enabled, so old
  records behave exactly as today), filterEnabledTools shared helper.
  quiz/ask_user_question are always-on FOR NOW (user-decided); the
  filtering is generic so future toggleables (question-tool variants,
  the read tool, …) need no factory change.
- createProberTools/createTeacherTools take enabled?: EnabledTools; the
  composition root (chat-handler, Platform's lane this wave) passes the
  resolved settings through. probe_done is re-added after the filter —
  always-on by construction (it ends the phase; a prober that can never
  conclude is a lockout).
- Prompts stay STATIC under toggles this wave (user-decided); standing
  principle recorded: tools ship self-contained usage guidance in their
  descriptions; system prompts reference only always-on tools.

Part 2 — phase-based question-writing principles (user-locked wording,
drafted with the user over three iterations):
- NEW teacher/21-question-principles.md (composes after 20-teaching):
  answerable-by-reasoning-from-the-session (the fairness rule that
  filters unanswerable questions without forbidding visible answers),
  never-just-reciting-your-words-back, PREFER problems that apply the
  knowledge (preference, not hard rule — honest recall is the fallback,
  not the habit), catch-confusion-early-and-nameable (wrong options as
  real misconceptions), answer-is-signal-not-verdict.
- prober/10-probe-doctrine.md: 'no rush' retired; quick-measurement
  directive in — every question moves an edge (bracket, narrow, or
  characterize); a bracketed strand is done.

Tests: TOGGLEABLE_TOOLS pin, prober/teacher filtering (absent = full,
false drops exactly that tool, probe_done survives), prompt pins (the
principles compose verbatim into the teacher prompt, the no-rush
framing is gone from the prober). 547/547, typecheck/lint/build green.

src/agent/tools.ts

@@ -5,6 +5,7 @@ import {
 	quizInputObject,
 	quizInputProblems,
 } from "../core/question-core.ts";
+import { filterEnabledTools, type EnabledTools } from "../core/pending.ts";
 import type { JobRecord } from "../core/contracts.ts";
 import type { JobStore } from "../core/ports.ts";
 import { jobTools, type DemoInput, type ResearchInput, type VisualInput } from "./tools/jobs.ts";
@@ -62,12 +63,28 @@ export const probeQuizTool: LhTool<any, any> = {
 	schema: probeQuizInputSchema,
 };
 
-/** The prober's tool set: probe quiz (no nodeId) + ask + probe_done. */
-export function createProberTools(ctx: ProbeDoneContext): Record<string, LhTool<any, any>> {
+/** The prober's tool set: probe quiz (no nodeId) + ask + probe_done.
+ *  `enabled` is the resolved settings seam (core/pending.ts): any tool
+ *  explicitly false is dropped from the registry. probe_done stays
+ *  always-on regardless — it is how the phase ENDS; a prober that can
+ *  never conclude is a lockout. */
+export function createProberTools(
+	ctx: ProbeDoneContext,
+	enabled?: EnabledTools,
+): Record<string, LhTool<any, any>> {
+	// probe_done is re-added AFTER the filter, always-on by construction — it
+	// is how the phase ENDS; a prober that can never conclude is a lockout.
+	const done = probeDoneTool(ctx);
 	return {
-		quiz: probeQuizTool,
-		ask_user_question: askUserQuestionTool,
-		...probeDoneTool(ctx),
+		...filterEnabledTools(
+			{
+				quiz: probeQuizTool,
+				ask_user_question: askUserQuestionTool,
+				...done,
+			},
+			enabled,
+		),
+		...done,
 	};
 }
 
@@ -131,12 +148,25 @@ export interface TeacherToolContext extends PlanToolContext {
 /**
  * Per-request teacher tool registry: teach questions (validated nodeId) +
  * plan tools + job tools, all closing over the ctx.
+ *
+ * `enabled` is the resolved settings seam (core/pending.ts EnabledTools —
+ * Platform's chat-handler wiring passes it through from settings). Today
+ * only the sub-agent tools (research/demo/visual) are user-toggleable, but
+ * the filtering is GENERIC: any name explicitly false is dropped, so
+ * future toggleables (question-tool variants, …) need no change here.
+ * Absent/undefined = enabled — tests and old records see today's set.
  */
-export function createTeacherTools(ctx: TeacherToolContext): Record<string, LhTool<any, any>> {
-	return {
-		quiz: teachQuizToolFor(ctx),
-		ask_user_question: askUserQuestionTool,
-		...planTools(ctx),
-		...jobTools({ chatId: ctx.chatId, jobStore: ctx.jobStore, spawn: ctx.spawn }),
-	};
+export function createTeacherTools(
+	ctx: TeacherToolContext,
+	enabled?: EnabledTools,
+): Record<string, LhTool<any, any>> {
+	return filterEnabledTools(
+		{
+			quiz: teachQuizToolFor(ctx),
+			ask_user_question: askUserQuestionTool,
+			...planTools(ctx),
+			...jobTools({ chatId: ctx.chatId, jobStore: ctx.jobStore, spawn: ctx.spawn }),
+		},
+		enabled,
+	);
 }