chore(skills): remove the stale test-pr-devnet skill

devnet-runner and devnet-log-review now cover everything this skill did
except building a branch-specific Docker image: node roster and image tag
selection, pause/unpause instability testing, and log analysis.

What remained had drifted from the current local setup. It hardcoded a
lean-quickstart path outside the repo, waited for exactly four `_0`
containers, and grepped zeam_0/ream_0/qlean_0, so every health check and
its whole result summary reported nothing against the all-ethlambda
validator config the local devnet uses now.

Author: MegaRedHand