Tell agents to format changed files with Prettier

[AbuBakkarBinAkmal]

Summary

• require agents to run Prettier 3 on every changed file it supports
• use the existing .prettierrc without command-line overrides
• limit formatting to explicit paths and require diff inspection to avoid unrelated churn

Historical context

The repository previously removed whole-repository formatter automation after it changed unrelated MDX. This keeps #5153 scoped to agent instructions and does not add CI auto-commits or another formatter dependency.

Validation

• npx --yes prettier@3 --write .prettierrc
• npx --yes prettier@3 --check .prettierrc
• compared the Prettier-normalized block with upstream/main; only the new guidance differs
• git diff --check upstream/main...HEAD

Closes #5153

[vercel]

@AbuBakkarBinAkmal is attempting to deploy a commit to the Temporal Team on Vercel.

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Pull request overview

Adds agent instructions for scoped Prettier 3 formatting using the repository configuration.

Changes:

• Requires formatting and checking supported changed files.
• Prevents broad formatting and unrelated churn.

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