feat: consume JSON output, add cli-version input

[shenxianpeng]

Summary

Makes the action robust and reproducible — the #1 engineering issue identified.

Changes

• JSON parsing — action now calls dm config --format json and parses with python3 -c, replacing fragile awk/sed text scraping
• cli-version input — allows pinning specific CLI versions for reproducible CI results
• README updates — recommends @v1 instead of @main, adds Reproducibility section

Example: before vs after

Before (awk/sed text parsing):

SCORE=$(echo "$OUTPUT" | awk '/Your score:/ {print $NF}')

After (JSON):

SCORE=$(echo "$JSON" | python3 -c "import sys,json; print(json.load(sys.stdin)['score'])")