feat: gate writer sections on a % teach: block (SP025)

Rewrite references/pedagogy.md around recite-vs-teach: reader model,
coverage density (sections_in is cite permission, not a to-do list),
intuition-before-formula three-beat, and paper-jump filling.

Every notes/sections/sec-*.tex must now answer gap / takeaway / jump /
omit above the first \section. lint.py checks presence only (it cannot
judge honesty); files opening with "% generated by" are exempt.

- scripts/lint.py: SP025 + is_writer_section / teach_gaps helpers
- tests/no-teach-block/: fixture with takeaway only, wired into test.sh
- examples/*: all 14 writer sections get real teach blocks
- SKILL.md, references/{agents,antipatterns,checklist}.md, DESIGN.md,
  assets/notes-template.tex: route writers and consistency agent
  through pedagogy.md

tests/no-teach-block/ledger.yaml

@@ -0,0 +1,20 @@
+schema: superpaper.ledger/v1
+retired_ids: []
+paper:
+  id: toy
+  title: Toy
+  source: {kind: excerpt}
+coverage:
+  mode: excerpt
+questions: []
+claims:
+  - {id: C1, text: core claim, kind: contribution, status: core}
+definitions: []
+assumptions: []
+lemmas: []
+symbols: []
+derivations: []
+figures: []
+evidence: []
+terms: []
+source_assets: []

tests/no-teach-block/notes/notes.tex

@@ -0,0 +1,5 @@
+\documentclass{article}
+\usepackage{amsmath}
+\begin{document}
+\input{sections/sec-01}
+\end{document}

tests/no-teach-block/notes/sections/sec-01.tex

@@ -0,0 +1,4 @@
+% takeaway: 有 takeaway，但缺 gap / jump / omit —— 闸门没走完
+\section{一次前向与损失}
+\splabel{C1}
+前向只负责预测；损失在预测之后单独比较。