feat: probe/teach agent split — explicit phase markers, position-only plan map

Diagnosis (2026-09-11 user testing, minimal-effort GLM): sessions stuck on
'plan' forever. Two causes: (1) the three-phase view read the stored tree's
currentNodeId, which nothing ever writes post author-once (2026-09-03) — a
structural dead seam; (2) the teacher stochastically left quiz.nodeId
untagged, and done-by-quiz-result semantics would have falsely completed
nodes anyway.

The redesign (docs/probe-teach-split.md) abolishes derived occupancy in
favor of explicit events:

- Two agents over one transcript: createProberAgent (quiz without a
  nodeId field, ask, probe_done) and createTeacherAgent (full toolset,
  plan_init onward). The chat-handler construction branch keys on a
  probe_done part in the transcript. MODELS.prober added.
- probe_done: server-side seam tool — carries the learner assessment
  (strands + free-form edge read), refuses while a question is open or
  the probe is already concluded, and queues a handoff steer through the
  generalized wake pipeline (steers now dedupe by id, not jobId; job
  finalization only for job steers). The teacher auto-wakes and authors
  the plan; the learner sees the assessment via the probe-done card.
- phases: probe ends at a concluded probe_done, plan ends at a
  successful plan_init — strictly sequential, no skipping, no legacy
  compatibility (data wiped at the pivot per doctrine).
- Plan map = position, not completion: derivePlanStatuses reads only
  tagged quiz CALLS (state/outcome feed nothing); current = latest
  tagged node with consecutive duplicates collapsed, visited = earlier
  distinct stays (a revisit stays in both — current wins at render).
  PlanTree is structure-only; currentNodeId/doneNodeIds and the dead
  mutation helpers are gone. No fallback current.
- Prompts split per agent (prompts/prober/, prompts/teacher/ over the
  trimmed shared base); graph-context injection removed entirely
  (shelved, docs/probe-teach-split.md §5); knowledge-graph context no
  longer reaches either agent.
- Plan blocks are full-only snapshots at plan_init (neighborhood
  machinery retired; the persistent DAG window is the future UI wave);
  usage capture unchanged.

Gates: tsc clean, 294/294 tests (phases/plan-tree/plan-blocks/question-
tools rewritten to the new semantics; probe-done + handoff-wake tests
added), lint, build. Live GLM smoke PASS: prober concluded with an honest
assessment, teacher auto-woke, authored a 10-node plan with the block
attached, then tagged checks on slope-instinct → secant derived current/
visited exactly as designed; grading honest both ways (driver off-by-one
proved a wrong answer grades false).

src/agent/prompts/teacher/50-probing-in-brief.md

@@ -0,0 +1,10 @@
+# The probe that came before — in brief
+
+This session's transcript opens with a probing phase, concluded by the probing agent before you took over: graded questions that bracketed his edge on every strand the lesson depends on, ending in the `probe_done` assessment (strand by strand: floor, ceiling, misconceptions, what to build from). Read the assessment and the probing conversation itself — the subtleties in how he answered carry signal the summary can't compress.
+
+The probing philosophy, in brief, so you know what those floors and ceilings mean:
+
+- The probe located the *edge* of his understanding per strand — the frontier between what he reliably gets right and what runs out — by binary search: escalate hard on success, narrow in on a miss. Nothing in it was a grade for him; it was frontier-mapping for you.
+- Your map (the plan) builds from his floors, starts at his ceilings, and routes around (or repairs) his misconceptions. That is the entire point of the probe's existence.
+
+If he answers a question mid-lesson in a way that makes you unsure where his edge actually sits on some strand, you can run a miniature probe yourself with `quiz` (adapt each question to the last answer; bracket floor and ceiling) — but never re-run the whole probing phase, and never re-open a concluded strand he's since demonstrated.