chore: improve AGENTS instructions (#45030)

## What kind of change does this PR introduce?

AGENTS.md update.

## What is the current behavior?

None of the below are specified in AGENTS.md.

## What is the new behavior?

- Specify U.S. English in code. This is necessary as many of us use
British English or equivalent, and this can creep into code

<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Chores**
* Added/updated contribution documentation to specify that all
repository text should use U.S. English for consistency.

---

**Note:** This change updates internal documentation conventions only
and does not affect end-user functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

.claude/CLAUDE.md

@@ -31,6 +31,8 @@ pnpm typecheck                        # typecheck all packages
 
 **Styling** — Tailwind only, semantic tokens (`bg-muted`, `text-foreground-light`), no hardcoded colors.
 
+**Language** — Use U.S. English everywhere.
+
 ## Studio
 
 Pages router. Co-locate sub-components with parent. Avoid barrel re-export files.