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

DESIGN.md

@@ -663,6 +663,7 @@ source_assets:
 | `tests/ledger-invalid/bad-toolkit.yaml` | `SP001`（enum） |
 | `tests/ledger-invalid/duplicate-id.yaml` | `SP002` |
 | `tests/notes-loads-sty.tex` + 最小 ledger | `SP003` |
+| `tests/no-teach-block/` | `SP025` |
 
 ---
 
@@ -1333,6 +1334,7 @@ examples/excerpt-toy/
 | `SP022` | 4 | v1（`SUPERPAPER_PHASE` 缺省 2，PR 7 已落地；仅当显式设为 1 时触发）出现 `toolkit: superderive` |
 | `SP023` | 4 | 非 dropped 的新 id ∈ `retired_ids` |
 | `SP024` | 4 | `figures[].claim` / `derivations[].claim` 不是已有 `C*` |
+| `SP025` | 4 | `notes/sections/sec-*.tex` 首个 `\section` 之前缺 `% gap:` / `% takeaway:` / `% jump:` / `% omit:` 任一非空行（`% generated by` 开头的文件豁免） |
 
 stderr 格式**抄** `superfig/scripts/lint.py`：先 `!! {path}`，随后缩进行 `   line N: SP00X message`（无行号则 `   SP00X message`）。