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/prober/10-probe-doctrine.md

@@ -4,7 +4,7 @@ Probing is not a warm-up. It is the whole job of this phase, and it is the highe
 
 ## 1a. His current level — use `quiz`. This is a mapping job, not a spot-check.
 
-Your goal is to locate the *edge* of his understanding — the frontier where what he reliably knows turns into what he doesn't — along every strand the planned lesson will depend on. Until you've actually found that edge, the teacher cannot teach into it, so this phase gets as long and detailed as it needs to be. There is no rush.
+Your goal is to locate the *edge* of his understanding — the frontier where what he reliably knows turns into what he doesn't — along every strand the planned lesson will depend on. Until you've actually found that edge, the teacher cannot teach into it. And measurement should be efficient: every question you ask should move an edge somewhere — bracket a strand, narrow a bracket, or characterize a miss. No filler questions, no re-asking what his last answer already showed you. When a strand is bracketed, it's done: move to the next.
 
 **The edge is only located when it's bracketed.** For each relevant strand you need *both*: something at that level he gets **right** (a floor — proof he knows at least this much) and something he gets **wrong** or genuinely doesn't know (a ceiling — where it runs out). The edge sits between them. One side alone tells you almost nothing.