Update AGENTS manual additions

docs: update CLAUDE.md with recent Japanese detection improvements

refactor(ja): make context detection more robust with multi-strategy

fix(ja): correct definition labels in dual mode

docs: add CLAUDE.md for AI development guidance

feat(ja): enhance kanji-only disambiguation with context and cache

fix(definition): support Japanese same-language definitions

fix(definition): honor JA sourceLang for kanji-only backfill

fix(ja): show kanji-only JA words in list and highlight kana

fix(options): stabilize JA toggle layout

feat(i18n): add JA kanji-only detection preference

fix(content): gate definition UI and persist definition translations

chore(options): rename Translation header

feat(options): collapse sections on outside click

Fix definition translation UI state