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/tensor-delegate/notes/sections/sec-01.tex

@@ -1,2 +1,7 @@
+% teach:
+% gap: 会写 $QK^\top$，但没想过收缩沿哪条边、$K^\top$ 要不要真的换面
+% takeaway: 这一节问的是形状对齐，不是模块拓扑
+% jump: none
+% omit: 摘录之外的全文结构
 \section{这篇论文在问什么}
 每头的 $Q$ 和 $K$ 沿哪一条边收缩，$K^\top$ 要不要真的换面？