Acceptance: executor-evidence-output-contract

Draft Acceptance

Issue exists: mvp/issues/archived/executor-evidence-attribution-hallucination.md.
OpenSpec change artifacts exist.
devflow tracking files exist.
OpenSpec validation passes.
Implementation updates Chat Executor prompt.
Implementation passes structured Executor output to Verifier.
Verifier prefers structured claims and still falls back safely.
Focused tests cover parsing, payload assembly, and unsupported confirmed claims.

This project is currently in proposal/design stage. Runtime code is intentionally not changed yet.