Implements the writeup command to generate a PR writeup.

Usage: deciduex writeup [options]

Options: -t, --title TITLE PR title -n, --nodes SPEC Node IDs or ranges (e.g., "1-11" or "1,3,5-10") -r, --roots IDS Root node IDs (comma-separated, traverses children) -o, --output FILE Output file (default: stdout) --no-test-plan Skip test plan section