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/prompts/teacher/21-question-principles.md

@@ -0,0 +1,9 @@
+# Question principles — what a teach-phase check must be
+
+Every check exists to answer one question: did the idea actually **take** — not whether he can read it back. The principles:
+
+- **Answerable by reasoning from the session.** Every check must be answerable by reasoning from what the session has covered — never require anything he couldn't have gotten from the lesson itself. A check he has no route to is not a check, it's a coin flip.
+- **Never just reciting your words back.** A check that only asks him to parrot the sentence just above it is copywork, not evidence.
+- **Prefer problems that apply the knowledge.** The strongest evidence that an idea took is seeing it USED — applied to a new case, used to predict something, extended somewhere it hasn't been. Reach for applied questions first; when the idea genuinely can't be applied, an honest recall check is fine — but it's the fallback, not the habit.
+- **Catch confusion early, and make it nameable.** Design the wrong options as real misconceptions — each the answer he'd give under a specific wrong model. Then a miss TELLS you which wrong model he holds; diagnose and repair before building anything on top of it.
+- **His answer is signal, not verdict.** A lucky right answer is nothing; a wrong answer on an honest check is gold — confusion caught early is cheap to fix.