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.

tests/fixtures/not-a-pdf.txt

@@ -0,0 +1 @@
+this is a text file, not a pdf

tests/fixtures/sample-3page.pdf

@@ -0,0 +1,54 @@
+%PDF-1.4
+1 0 obj
+<< /Type /Catalog /Pages 2 0 R >>
+endobj
+2 0 obj
+<< /Type /Pages /Kids [3 0 R 5 0 R 7 0 R] /Count 3 >>
+endobj
+3 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 4 0 R /Resources << /Font << /F1 9 0 R >> >> >>
+endobj
+4 0 obj
+<< /Length 89 >>
+stream
+BT /F1 14 Tf 72 720 Td (Calculus page: the integral of x dx is x squared over two) Tj ET
+endstream
+endobj
+5 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 6 0 R /Resources << /Font << /F1 9 0 R >> >> >>
+endobj
+6 0 obj
+<< /Length 82 >>
+stream
+BT /F1 14 Tf 72 720 Td (Physics page: force equals mass times acceleration) Tj ET
+endstream
+endobj
+7 0 obj
+<< /Type /Page /Parent 2 0 R /MediaBox [0 0 612 792] /Contents 8 0 R /Resources << /Font << /F1 9 0 R >> >> >>
+endobj
+8 0 obj
+<< /Length 60 >>
+stream
+BT /F1 14 Tf 72 720 Td (Chemistry page: H2O is water) Tj ET
+endstream
+endobj
+9 0 obj
+<< /Type /Font /Subtype /Type1 /BaseFont /Helvetica >>
+endobj
+xref
+0 10
+0000000000 65535 f 
+0000000009 00000 n 
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+trailer
+<< /Size 10 /Root 1 0 R >>
+startxref
+953
+%%EOF