feat(T2b): sources v0 — read tool + pdf-reader sub-agent

Learner source files, on the teacher's own terms (nothing auto-injects):

- read tool (src/agent/tools/read.ts): pi-read port — path/offset/limit,
  2000-line/50KB truncation with use-offset continuation notices, image
  files (jpg/png/gif/webp/bmp) returned as native image content parts
  (new LhToolImageResult escape hatch in lh-tool.ts). Always-on in both
  the prober and teacher registries (user-decided 2026-09).
- pdf-reader sub-agent (JobKind 'pdf', toggleable, ack-and-steer): triage
  via pdf_info (pdfjs text layer: per-page text/image/math-density),
  targeted pdf_extract, regex pdf_search (hits count as consultation),
  and pdf_render — pages painted by pdfjs onto a canvas shim in headless
  Chrome (the pdfium viewer doesn't paint headless), screenshot fed back
  image-in-the-loop (visual-maker idiom, providerSafePng applied). The
  unread-finish gate bounces a zero-consultation brief once. Teacher gets
  a text brief with page citations only; renders never leave the loop.
- MODELS.pdfReader = qwen/qwen3.8-flash (user override 2026-09-19: the
  look-at-renders workload wants the vision-proven model; catalog listed
  glm-5.3-flash as image-capable but qwen is the known-good daily driver).
- UI: PdfCompleteBadge + PdfBriefCard (steer-card, job-card), dispatch
  card + activity-dock kind entries (type-forced, flagged for UI review).
- Prompts: 'Learner source files' section in teacher/30-jobs.md; read
  line in prober/10-probe-doctrine.md. Doctrine rides tool descriptions
  per the self-contained-tools principle.

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

@@ -21,4 +21,6 @@ Find out what he actually wants taught. With a subject he doesn't know yet, the
 
 ## How your questions arrive
 
-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.
+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.

src/agent/prompts/teacher/30-jobs.md

@@ -31,3 +31,17 @@ message: say so plainly and continue with what you can establish, flagging the u
 A job can also be **aborted by the learner** — you'll get a one-line "was aborted"
 message instead of the result. Acknowledge it in one sentence and continue teaching
 without it (offering to re-dispatch is fine if the learner wants the result).
+
+## Learner source files
+
+The learner may paste **file paths** into the chat — course PDFs, notes, screenshots, code.
+Reading them is on your own terms; nothing is injected automatically.
+
+- For quick looks at plain text or image files, call `read` directly (it returns the text or
+  shows you the image; large files come back in slices with continuation offsets).
+- For PDFs, dispatch a `pdf` job instead: it runs a background reader (triage, targeted
+  extraction, visual page reading) and returns a **brief with page citations** as a new
+  message — the same fire-and-forget discipline as research above. Give it the path exactly
+  as the learner gave it plus the specific question you need answered.
+- Ask for the path when the learner references a file they haven't given you — never ask them
+  to paste file contents into the chat.