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/derive-delegate/notes/sections/sec-03.tex

@@ -1,2 +1,7 @@
+% teach:
+% gap: 不知道什么时候该出 superderive 图、什么时候 align 就够
+% takeaway: $\le 4$ 步且没有消项或代入要看见时，留在 align
+% jump: none
+% omit: superderive 的完整语法（见子仓库 SKILL.md）
 \section{总结与延伸}
 四步以内、没有消项或代入要画出来时，仍然用 \texttt{align}。