feat(i18n): set up glossary (#2198)

This pull request introduces the initial glossary for the project to
improve the translation quality.

Implements #2148.

### TODOs

- [x] Add the definition YAML
- [x] Add a generator to produce Crowdin-compatible (TBX v3) format
- [x] AGENTS.md
- [ ] ~Workflow~ (Split to another PR)
- [ ] ~Skills~ (Same as above)

AGENTS.md

@@ -143,6 +143,37 @@ Concise but detailed reference for contributors working across the `moeru-ai/air
 
 - Add/modify translations in `packages/i18n`; avoid scattering i18n across apps/packages.
 
+### Glossary
+
+`packages/i18n/glossary/terms.yaml` gives the approved English term for each product concept.
+`pnpm -F @proj-airi/i18n glossary:build` writes the TBX file that Crowdin imports. `schema.ts`
+documents each field.
+
+- Read `terms.yaml` before you write or change a string that a user sees. Use the term it gives.
+- Do not edit `packages/i18n/glossary/translations/`. The next Crowdin download replaces it.
+- Take terms from the interface. Search the documentation to confirm a term, not to find one.
+
+**Add a term when it prevents one of two failures:**
+
+- A translator who reads only the source string chooses the wrong word.
+- Two translators choose different words, and both words are correct.
+
+**Do not add a term when:**
+
+- It is code. A translated control token, JSON key, file name, or package name breaks the
+  application, and the application shows no error.
+- It is a common English word with its usual meaning, such as "Speed" or "Volume".
+- Its parts each have an entry and it says no more than its parts, such as "VRM model".
+- One feature uses it, and only two translators could disagree about it. But keep a rare term if a
+  translator can get it wrong: `Tachie` occurs two times and is the strongest entry in the file.
+
+`terms.yaml` holds no rationale, because each field must map to a TBX element. Give the reason in
+the pull request.
+
+**Write a definition in one sentence.** Describe the thing, not the word, and put a negative rule in
+the `note`. Keep each sentence short and active: translators read them, and most do not read English
+as a first language.
+
 ## CSS/UNO
 
 - Use/extend UnoCSS shortcuts in `uno.config.ts`.