sync: update from private repo (c2c10e0)

CONTRIBUTING.md

@@ -65,6 +65,18 @@ npx vitest run src/engine/tools/core.test.ts   # a single file
 - DB schema changes: update `src/db/schema.sql` and add an idempotent migration in
   `src/db/migrate.ts`.
 
+## Translations
+
+Documentation is **English-first**, with a Japanese sibling using a `.ja.md`
+suffix (e.g. `README.md` / `README.ja.md`, `docs/docker.md` / `docs/docker.ja.md`).
+English is canonical: a `.md` page is the source of truth and its `.ja.md`
+sibling is the translation.
+
+If you change an English page, update the `.ja.md` sibling when you can. If you
+can't, note in the PR that the translation needs updating — a stale translation
+should not block an English fix. New docs should land in English first; a
+translation can follow in a later PR.
+
 ## Architecture
 
 See `AGENTS.md` for a contributor-oriented architecture overview and