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

examples/excerpt-toy/notes/sections/sec-05.tex

@@ -1,2 +1,7 @@
+% teach:
+% gap: 学完这一条机制，不知道同样的流程怎么套到一篇完整论文
+% takeaway: 先用 ledger 钉住 claim，再按路由决定出不出图
+% jump: none
+% omit: 摘录没覆盖的实验与消融
 \section{总结与延伸}
 摘录只保留一条机制：一次前向加侧路损失。更长的论文用 ledger 把 claim 钉住，再按路由出图。