fix: honor precise Flow execution outcomes

[abrichr]

Summary

• make Flow's precise execution outcome authoritative for MCP run and report results
• ensure COMPLETED_UNVERIFIED and ROLLED_BACK never become MCP success
• fail closed on malformed, contradictory, or envelope-inconsistent reports
• preserve legacy reports that only expose the boolean success field
• keep all public projections count-only and use one fixed message mapping

Verification

• 89 tests passed
• Ruff passed for src, tests, and scripts
• diff check passed

The output change is additive: execution_outcome appears only when the persisted Flow report provides a recognized precise value. Existing status and success fields remain compatible.