ci: add cron to auto remove stale branches

[bowenli86]

Motivation:

many stale branches lives in the repo and never got cleaned up. to balance dev convenience vs being organized/legible, this pr implements a CI workflow to identify and remove stable branches

Modifications:

add a ci workflow that

• scan stale branches that not hv new commits since 90 days ago, and notify owner if there's one
• if still no changes coming in, delete the stable branch 7 days later

following branches and regex are excluded

• main
• release-dev/*
• v*.*.* (release branches)

and can be changed by modifying exempt-branches-regex config in the yml

Result:

successfully dry run
https://github.com/Layr-Labs/eigenlayer-contracts/actions/runs/14741452510/job/41380210924