Add structured artifact index for workspace outputs

[Zefan-Cai]

Summary

• add a run-level artifact_index.json for structured indexing of workspace/data, workspace/results, and workspace/figures
• capture basic inferred schema metadata for JSON, JSONL, CSV/TSV, figures, and declared sidecar schemas
• feed the artifact index into later-stage prompt context and the writing manifest instead of relying only on ad hoc directory scans
• document the new index in the README roadmap and run layout

Covered TODO

• Completes TODO 5: result schema and artifact indexing

Validation

• python -m py_compile main.py src/.py tests/.py
• python -m unittest discover -s tests -v