Move disk-usage functionality from Makefile to a dedicated shell script
for better maintainability and consistency with other scripts.
Changes:
- Create scripts/disk-usage.sh with all disk usage logic
- Simplify Makefile disk-usage target to call the script
- Add environment loading and color formatting to script
- Fix duplicate output by adding head -1 to grep chain
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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
