Add suite-level runner for scenario and trace directories

[mertsatilmaz]

Context

The harness can run one scenario at a time. Teams will quickly want a suite mode that runs a directory of scenarios against corresponding traces or targets and produces one aggregate result.

Acceptance criteria

• Design a minimal suite command or flag that keeps single-scenario behavior intact.
• Support trace-file based suites first.
• Emit per-scenario result JSON and an aggregate summary.
• Exit behavior should compose with the CI fail-on-result option.
• Add tests for partial failures and missing trace mappings.
• Document expected directory conventions.

Useful files

• src/agent_harness/cli.py
• src/agent_harness/runner.py
• tests/test_cli.py
• docs/ci-github-actions.md