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

assets/notes-template.tex

@@ -13,7 +13,9 @@
 % \section{推导链一览}
 % \section{图表清单}
 %
-% Formula: Chinese motive, then \[ / align, then a flat symbol list.
+% Teach, don't recap. Each writer section starts with
+% % teach: gap / takeaway / jump / omit  (references/pedagogy.md).
+% Formula: intuition off-stage, then \[ / align, then a flat symbol list.
 % Keep figures outside knowledgebox / importantbox / warningbox / quotebox.
 % Do not \usepackage{superfig|supertensor|superderive}.