docs: split AGENTS.md into core policy and on-demand playbooks

2602.12670 and 2602.11988 findings favoring curated, minimal, modular guidance over comprehensive always-on context

docs/agent-playbooks/translations.md

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+# Translations Workflow
+
+This project uses i18next translation files in `public/translations/{lang}/default.json`.
+
+## Rule
+
+Do not manually edit every language file. Use `scripts/update-translations.js`.
+
+## Add or Update a Key
+
+1. Create a temporary dictionary file, e.g. `translations-temp.json`:
+
+```json
+{
+  "en": "English text",
+  "es": "Spanish text",
+  "fr": "French text",
+  "de": "German text"
+}
+```
+
+2. Apply the translation map:
+
+```bash
+node scripts/update-translations.js --key my_new_key --map translations-temp.json --include-en --write
+```
+
+3. Delete the temporary dictionary file.
+
+## Other Useful Commands
+
+```bash
+# Copy a key from English to all languages (dry run then write)
+node scripts/update-translations.js --key some_key --from en --dry
+node scripts/update-translations.js --key some_key --from en --write
+
+# Delete a key from all languages
+node scripts/update-translations.js --key obsolete_key --delete --write
+
+# Audit for unused translation keys
+node scripts/update-translations.js --audit --dry
+node scripts/update-translations.js --audit --write
+```