test(contract): cover five demo schemas

[stacknil]

Summary

• add schemas/telemetry_summary.schema.json for telemetry-window demo summaries
• repoint schemas/investigation_summary.schema.json to the config-change investigation summary artifact
• add schemas/cloud_iam_findings.schema.json and schemas/cloud_iam_summary.schema.json for the Cloud IAM demo
• update the evidence-pipeline contract docs and tests so schema coverage spans all five demos and the named artifacts

Validation

• pytest tests/test_evidence_pipeline_schemas.py tests/test_reviewer_docs.py tests/test_markdown_links.py
• python scripts/regenerate_artifacts.py --check
• pytest
• git diff --check

Safety

• schema/docs/test-only change
• no new demo surface
• privacy scan on touched files found no account IDs, AWS key prefixes, local paths, or local usernames