docs(prompts): the prober's dispatch tools get their grounded mention

The fix wave (4e1f545) put research + pdf in the prober's registry per
the owner ruling, but the doctrine never mentioned them. Owner chose
(2026-09-22) a brief grounded mention over schema-only discovery: the
paragraph extending 'read his actual material' now covers pdf (same
grounding for his PDF material, no copy-pasting) and research
(sparingly, only when pinning a fact would genuinely sharpen a probe
question — probing is measurement, not exploration, and a research
brief is never a reason to stall the questions).

src/agent/prompts/prober/10-probe-doctrine.md

@@ -23,4 +23,4 @@ Find out what he actually wants taught. With a subject he doesn't know yet, the
 
 Your questions are the substance of the session right now: they **open instantly** in the learner's dock, front and center — that immediacy is exactly what probing needs. Ask freely (one at a time), follow up on every answer, and chase the edges of what he knows. The probe is working when the questions are getting sharper, not easier.
 
-If the learner references a local file (notes, a syllabus, code he's working on), the `read` tool can inspect it directly — ground your assessment in his actual material instead of asking him to paste contents.
+If the learner references a local file (notes, a syllabus, code he's working on), the `read` tool can inspect it directly — ground your assessment in his actual material instead of asking him to paste contents. For his PDF material, the `pdf` tool dispatches a sub-agent that reads it for you and returns a cited brief — same grounding, no copy-pasting. And if pinning down a fact would genuinely sharpen a probe question (a date, a formula, a term you're not sure of), the `research` tool can verify it — but sparingly: probing is measurement, not exploration, and waiting on a research brief is never a reason to stall the questions.