📝 document yarn format --write in AGENTS.md

[thomas-lebeau]

Motivation

AGENTS.md listed yarn format under "Format code", but the script is prettier --check .: it only reports issues and never modifies files. Agents (and humans) following it after a bulk edit believe the code was formatted when it wasn't, and the format CI job fails afterwards — this happened on #5022. Agents also tend to fall back to calling yarn prettier directly, which can diverge from what CI checks.

Changes

Documentation only, root AGENTS.md "Key Commands":

• Relabel yarn format as a check (--check, does not modify files).
• Add yarn format --write to fix formatting (Yarn appends the flag; Prettier 3 accepts --check --write, rewrites the files and lists them).
• Add a short note to always go through the yarn format script rather than yarn prettier, and to check its output after scripted/bulk edits.

Test instructions

printf 'const   x = {a:1}\n' > tmp-probe.ts
yarn format          # reports tmp-probe.ts, file unchanged
yarn format --write  # "Code style issues fixed", file rewritten
rm tmp-probe.ts

Checklist

• Tested locally
• Tested on staging
• Added unit tests for this change.
• Added e2e/integration tests for this change.
• Updated documentation and/or relevant AGENTS.md file