Add docs/decisions/ with a convention README and back-fill two
decisions that previously had no in-repo home: merging PRs with
rebase (ADR 0001) and the agent-identity claimed-not-vouched trust
posture from PRD 0027 (ADR 0002). Point docs/INDEX.md at it.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
