Optimise GitHub Actions

Billed time and wall-clock time are different numbers, and most CI waste lives between them. A 5-way matrix that finishes in 2 minutes bills 10 or more. A 5-second job bills a whole minute. A merge to main can re-run every suite the pull request already passed. So measure first, then cut the biggest items, and keep the checks that guard what you ship.

Speed and cost usually move together, but not always. Caches and removing duplicate work help both. Splitting a long job into parallel shards makes CI faster and more expensive, because each shard pays setup again. Merging small jobs does the reverse. Ask which one the user cares about, or say which way each change goes.

1. Measure

Run the script in this skill's folder:

node <skill-dir>/scripts/measure.mjs OWNER/REPO --days 14 --out jobs.json

It needs gh with read access and nothing more. It prints: