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/core/pending.ts

@@ -30,6 +30,38 @@ export const QUESTION_TOOLS = ["quiz", "ask_user_question"] as const;
 /** Tools fulfilled by background sub-agent jobs (W2/W6). */
 export const JOB_TOOLS = ["research", "demo", "visual"] as const;
 
+// ─── Tool availability (2026-09 settings seam) ─────────────────────────────
+
+/** The tools a user may toggle off in settings (Wave 2 T2a, user-decided
+ *  2026-09-15): for now only the SUB-AGENT dispatch tools — quiz,
+ *  ask_user_question (and the read tool, when it lands) are always on;
+ *  the long-term vision is that everything becomes toggleable as new
+ *  tool variants (e.g. alternative question tools) ship, so the factory
+ *  seam below is GENERIC: it honors any tool name it receives. New
+ *  toggleables join this list when settings exposes them.
+ *  The names are the MODEL-FACING tool names (research, not researcher). */
+export const TOGGLEABLE_TOOLS = ["research", "demo", "visual"] as const;
+
+/** The resolved settings → factory seam shape (Platform wires chat-handler
+ *  to pass this through from the settings record; storage and defaults are
+ *  Platform's lane). JSON-native: a partial record keyed by tool name.
+ *  Absent key / undefined = enabled — old records behave exactly as today.
+ *  Any explicit `false` removes that tool from the registry, whatever its
+ *  name — the machinery stays generic for future toggleables. */
+export type EnabledTools = Partial<Record<string, boolean>>;
+
+/** Drop every tool the resolved settings mark as disabled. The single
+ *  filtering site every factory shares (prober/teacher). */
+export function filterEnabledTools<T>(tools: Record<string, T>, enabled?: EnabledTools): Record<string, T> {
+	if (!enabled) return tools;
+	const out: Record<string, T> = {};
+	for (const [name, tool] of Object.entries(tools)) {
+		if (enabled[name] === false) continue;
+		out[name] = tool;
+	}
+	return out;
+}
+
 function findPending(messages: UIMessage[], names: readonly string[]): PendingToolCall | null {
 	for (let m = messages.length - 1; m >= 0; m--) {
 		const msg = messages[m]!;