docs: add context budget guidance

[caioribeiroclw-pixel]

Adds a small note that AGENTS.md is part of the agent context budget, so the default guidance should stay focused and link out to deeper docs when details are task-specific.

Why:

• Recent discussion around AGENTS.md effectiveness is making context cost more visible.
• The current page says "anything you'd tell a new teammate belongs here too", which is useful but can nudge people toward long up-front overviews.
• This keeps the format simple while encouraging durable, task-relevant instructions.

Checks:

• corepack pnpm exec tsc --noEmit
• corepack pnpm run build
• git diff --check

[vercel]

@caioribeiroclw-pixel is attempting to deploy a commit to the openai Team on Vercel.

A member of the Team first needs to authorize it.

[Pjrich1313]

Jump and run diff merge into wave for audit

[caioribeiroclw-pixel]

Thanks for taking a look. I re-ran the PR locally after the review note:\n\n- git diff --check origin/main...HEAD clean\n- corepack pnpm exec tsc --noEmit passed\n- corepack pnpm run build passed\n\nThe current Vercel failures look like deployment authorization for the upstream team, not a patch failure. The diff is intentionally copy-only and small: one focused context-budget step + one FAQ entry.\n\nIf you meant a specific audit/merge target by "diff merge into wave", point me at it and I can adjust the PR.