Add explanations

CLAUDE.md

@@ -78,7 +78,10 @@ data/ ──[Stage 2: amateurfunk_anki.py]──► anki/
    the front, the displayed position of the correct answer on the
    back. Inline `$...$` LaTeX is converted to MathJax `\(...\)`
    delimiters; the catalog's safe inline markup (`<u>...</u>`) is
-   preserved.
+   preserved. If the question's number has an entry in
+   `explanations.json` (see EXPLANATIONS.md), an English explanation
+   block is appended to the back; a "low confidence" badge shows for
+   entries with `confidence < 7`.
 4. Hand-roll the v11 Anki collection (SQLite + JSON config) and
    package it as a `.apkg` ZIP with deterministic timestamps. Same
    input → byte-identical output across runs.
@@ -103,6 +106,11 @@ SVG dark-mode handling, schema choices) live in `DESIGN.md` §7.
 
 ## Working on this repo
 
+- `EXPLANATIONS.md` — the editorial contract for agents asked to add
+  or improve per-question explanations. The schema, the workflows
+  ("explain everything unexplained", "improve everything below
+  confidence 7"), and the source/confidence guidance live there.
+  `explanations.json` is an empty `{}` until agents populate it.
 - Start from `DESIGN.md` — it has the JSON schema, the question/answer
   conventions (answer A is always correct upstream → consumers shuffle
   before display), the LaTeX-in-questions caveat, the exam-structure