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

scripts/test.sh

@@ -76,6 +76,7 @@ expect_work "$ROOT/tests/loads-sty" SP003
 expect_work "$ROOT/tests/missing-request" SP012
 expect_work "$ROOT/tests/unlabeled-core" SP020
 expect_work "$ROOT/tests/missing-include" SP021
+expect_work "$ROOT/tests/no-teach-block" SP025
 
 echo "==> compile"
 for dir in \