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-04.tex

@@ -1,3 +1,8 @@
+% teach:
+% gap: 会算内积，但不知道维数一涨为什么会把后面的非线性弄坏
+% takeaway: 除 $\sqrt{d}$ 是把方差按回 1 的改写，不是新算子
+% jump: 摘录直接写下 $1/\sqrt{d}$，没说维数涨会让点积方差跟着涨
+% omit: 数据集、超参、硬件
 \section{一次前向与损失}
 内积的方差会随维数 $d$ 涨。为了不让后续非线性饱和，要把点积除掉 $\sqrt{d}$。