- add output column to ScenarioRunStepEntity; exec step return value is JSON-serialised and stored
- expose helpers.getStepOutput(order) in exec code; negative order is relative to current step
- add get_scenario_run, wait_for_scenario_run, export_scenario, import_scenario MCP tools
- move Architecture, Key descriptors, MCP tools, Scenario, Development docs to docs/
- delete CONTRIBUTING.md (replaced by docs/development.md)
