Add versioned pipeline scenario contracts

[auschoi96]

Summary

• add four versioned, runner-neutral pipeline scenario contracts
• validate the contracts with strict Pydantic models and a committed JSON Schema
• load the existing pipeline scenarios through the shared contracts while preserving prompts, capabilities, stages, and timeouts
• make the demo-generator skill self-locate when project path aliases are unavailable
• keep all scenario assets outside the deployed generator app and shipped skill

Verification

• uv run pytest tests/evaluation -q
• uv run sb-eval cases validate
• uv run python -m evaluation.schema_generator --check
• uv lock --check
• git diff --check origin/main...HEAD