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

references/checklist.md

@@ -1,6 +1,8 @@
 # Delivery checklist
 
 - Ledger written first; every core claim has `\splabel`.
+- Notes teach from a reader gap, not the paper's section order (`references/pedagogy.md`).
+- Every `sec-*.tex` opens with the four `% teach:` lines (`SP025`).
 - Formula three-beat present wherever display math appears.
 - Each figure toolkit matches `scripts/router.py`; mixed-class rows were split.
 - Notes do not `\usepackage{superfig|supertensor|superderive}`.