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/pipeline-delegate/notes/sections/sec-02.tex

@@ -1,3 +1,8 @@
+% teach:
+% gap: 容易把损失画成前向链条上的下一个方块
+% takeaway: 损失是侧路，不是主路上的一站
+% jump: fixture 的框图没说清 $L$ 是挂在 $\hat y$ 之后还是在链条之内
+% omit: 重复摘要的贡献条目
 \section{主张与贡献}
 \splabel{C1}
 一次前向是 $x\to f_\theta\to\hat y$。损失在预测之后单独比较。