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.

components/activity-dock.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { createContext, useCallback, useContext, useEffect, useMemo, useRef, useState } from "react";
-import { ActivityIcon, BookOpenIcon, BotIcon, PenToolIcon, PlayIcon, SearchIcon, SquareIcon, WaypointsIcon } from "lucide-react";
+import { ActivityIcon, BookOpenIcon, BotIcon, FileTextIcon, PenToolIcon, PlayIcon, SearchIcon, SquareIcon, WaypointsIcon } from "lucide-react";
 import { cn } from "@/lib/utils";
 import type { CallUsage, JobKind, JobRecord } from "@/src/core";
 import { CONTEXT_LIMITS, jobCost } from "@/src/core";
@@ -232,6 +232,7 @@ const KIND_ICON: Record<JobKind, typeof SearchIcon> = {
 	research: SearchIcon,
 	demo: PlayIcon,
 	visual: PenToolIcon,
+	pdf: FileTextIcon, // sources v0 — type-forced entry (JobKind grew "pdf")
 	"graph-context": BookOpenIcon,
 };
 
@@ -347,9 +348,11 @@ function ActivityBlobRow({
 					? "Demo builder"
 					: blob.jobKind === "visual"
 						? "Figure maker"
-						: blob.jobKind === "graph-context"
-							? "Graph context"
-							: "Sub-agent";
+						: blob.jobKind === "pdf"
+							? "PDF reader"
+							: blob.jobKind === "graph-context"
+								? "Graph context"
+								: "Sub-agent";
 	return (
 		<div
 			data-testid={`activity-blob-${blob.process}`}