Standardize Evidence Artifacts and Schema

## Goal
Standardize the artifact set emitted by all onboarded repositories.

## Why
Developers and governance tooling need consistent machine-readable evidence across repos.

## Required Artifact Contract
- `execution_lane` (`oss_default` | `accelerated`)
- `policy_decision`
- `test_summary`
- `quality_or_traceability_metrics`
- `dependency_manifest_summary`
- `license_policy_result`
- `reproducibility_metadata` (rerun id, tool versions, environment hash)

## Tasks
- Define artifact manifest and schema versions.
- Reuse/align with existing docs-as-code schema patterns.
- Validate schema compatibility in CI.
- Document retention and discoverability.
- Define compatibility policy for schema evolution.

## Docs-as-code pilot tasks
- Extend current metrics artifacts with required provenance fields.
- Validate artifact schemas in CI before policy decision.
- Ensure accelerated-lane artifacts remain verifiable by OSS validators.

## Done When
- Pilot repos emit the same artifact contract.
- CI rejects malformed or incomplete artifact payloads.
- Schema version policy is documented and enforced.

Parent: https://github.com/eclipse-score/score/issues/2827