docs: migrate sponsor link to canonical UTM convention (#29)

Update the README sponsor link's UTM params to match the canonical
"Modem-owned repo README" row in the GTM UTM Tagging Strategy
(updated 2026-05-18): utm_campaign=oss_slop_scan (snake_case) and
utm_content=readme_footer. Aligns with the convention applied to
recently-added sponsor blocks in modem-dev/baudbot and
modem-dev/glance-agent-plugins.

Linear: GTM-32

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>