Support parallel scenario execution

[aviralgarg05]

Summary

Implement parallel scenario execution to speed up evaluation suites with many independent scenarios.

Tasks

• Add --parallel / -j flag to CLI (default: 1).
• Use concurrent.futures.ThreadPoolExecutor or ProcessPoolExecutor.
• Ensure thread-safe trace logging and result aggregation.
• Add tests for parallel execution correctness.

Acceptance criteria

• agentunit eval scenarios/ -j 4 runs 4 scenarios concurrently.
• Results are identical to sequential execution.
• No race conditions in trace or metrics collection.