feat: add opencode browser click challenge harness
- Challenge prompt requiring workflow/deployment/run evidence
- CLI harness (run_opencode_trials.py) for running N agent trials
- Classification: success, workflow_not_used, run_failed, timeout, parse_error, unknown
- 8 unit tests covering build/parse/classify/path logic
- README with usage docs and optional Playwright MCP attachment
- Evidence index and roadmap updated
- Plan archived to historical/
