fix: replace bc with awk in retry sleep calculation

[milo-oaklight]

PR #3 nit: sleep line used bc for arithmetic but skill requires only declares curl/jq. Replaced with awk which is ubiquitous.

# before (needs bc)
sleep "$(echo "scale=1; $wait_ms / 1000 + 0.5" | bc)"

# after (awk, always available)
sleep "$(awk "BEGIN{printf \\"%.1f\\", $wait_ms/1000+0.5}")"