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-02.tex

@@ -1,3 +1,8 @@
+% teach:
+% gap: 知道要除 $\sqrt{d_k}$，但看不见方差是从哪一步冒出来的
+% takeaway: 缩放来自点积方差随 $d_k$ 增长，是改写不是新算子
+% jump: fixture 直接给出结论，没展开方差那一步
+% omit: 其他归一化方案的对比
 \section{主张与贡献}
 \splabel{C1}
 点积方差随 $d_k$ 涨。除掉 $\sqrt{d_k}$ 是改写，不是新算子。