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/artifacts/steer-card.tsx

@@ -1,10 +1,10 @@
 "use client";
 
 import { CheckIcon, CircleX } from "lucide-react";
-import type { ResearchResult, SubagentArtifactData, TeacherUIMessage } from "@/src/core";
+import type { PdfResult, ResearchResult, SubagentArtifactData, TeacherUIMessage } from "@/src/core";
 import { isSubagentFailure } from "@/src/core";
 import { DemoOutputCard } from "@/components/artifacts/demo-card";
-import { ResearchBriefCard } from "@/components/artifacts/job-card";
+import { PdfBriefCard, ResearchBriefCard } from "@/components/artifacts/job-card";
 import { VisualOutputCard } from "@/components/artifacts/visual-card";
 
 /**
@@ -48,6 +48,12 @@ export function SubagentSteerCard({ message }: { message: TeacherUIMessage }) {
 			) : (
 				<ResearchCompleteBadge result={data.result as ResearchResult} />
 			);
+		case "pdf":
+			return isSubagentFailure("pdf", data.result as PdfResult) ? (
+				<PdfBriefCard output={data.result} />
+			) : (
+				<PdfCompleteBadge result={data.result as PdfResult} />
+			);
 		case "demo":
 			return <DemoOutputCard output={data.result} />;
 		case "visual":
@@ -68,6 +74,32 @@ function AbortedBadge({ kind }: { kind: string }) {
 	);
 }
 
+/** Quiet one-liner, the research-badge idiom ported to PDF: the brief is
+ * too long to inline by default and the teacher's wake turn weaves it in;
+ * the badge marks delivery, the collapsed details carries the full text
+ * + consulted pages. */
+function PdfCompleteBadge({ result }: { result: PdfResult }) {
+	return (
+		<details data-testid="pdf-complete-badge" className="group w-full text-xs text-muted-foreground">
+			<summary className="flex cursor-pointer select-none list-none items-center gap-2 [&::-webkit-details-marker]:hidden">
+				<CheckIcon className="size-3.5" />
+				<span className="font-medium uppercase tracking-wide">PDF</span>
+				<span>reading complete</span>
+				{result.pagesConsulted.length > 0 && (
+					<span>
+						· {result.pagesConsulted.length} page{result.pagesConsulted.length === 1 ? "" : "s"} consulted
+					</span>
+				)}
+				<span className="ml-1 text-xs text-muted-foreground/70 group-open:hidden">view brief</span>
+				<span className="ml-1 hidden text-xs text-muted-foreground/70 group-open:inline">hide brief</span>
+			</summary>
+			<div className="mt-2">
+				<PdfBriefCard output={result} />
+			</div>
+		</details>
+	);
+}
+
 /** Quiet one-liner, the dispatch-settled idiom — the lesson lives in the
  * teacher's wake turn; the badge just marks delivery. The persisted brief
  * rides along in a collapsed <details> (the wave-1 "view full research"