fix(bmad-product-brief): tighten update/validate rules and eval expectations

- Update: audit trail (decision-log + addendum) is now mandatory before
  modifying brief.md in headless mode; distillate regeneration is required
- Validate: always emits offer_to_update in headless JSON output
- Headless: added validate example to JSON status block docs
- Evals B8: add 900s timeout, replace hard 30%-smaller check with
  meaningful-condensation expectation
- Eval B9: sharpen right-sized expectation wording