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-02.tex

@@ -1,3 +1,8 @@
+% teach:
+% gap: 把 $K^\top$ 当成记号上的装饰，不当成一次真实的换面
+% takeaway: 打分沿 $d_h$ 收缩，收缩边必须等长
+% jump: fixture 只写 $QK^\top$，没说明转置是物理换面而非书写约定
+% omit: 多头拼接与输出投影
 \section{主张与贡献}
 \splabel{C1}
 打分是 $(T\times d_h)(d_h\times T)\to(T\times T)$。$K^\top$ 必须物理换面，收缩边等长。