chore(skills): remove the stale test-pr-devnet skill (#564)

Removes `.claude/skills/test-pr-devnet` and repoints the two `CLAUDE.md`
references at the skills that replaced it.

## Why

`devnet-runner` and `devnet-log-review` already cover what this skill
did:

| test-pr-devnet did | now covered by |
|---|---|
| Start a multi-client local devnet | `devnet-runner` (node roster,
timeout, automatic log dump) |
| Patch `client-cmds/*-cmd.sh` image tags | `devnet-runner` |
| pause/unpause sync testing |
`devnet-runner/references/instability-testing.md` |
| Status checks and log greps | `devnet-log-review` scripts |

What was left had drifted from the current local setup:

- It hardcoded
`LEAN_QUICKSTART=/Users/mega/lean_consensus/lean-quickstart`, while
`devnet-runner` expects `lean-quickstart` at the repo root.
- Its readiness loop waited for exactly four `_0` containers and its
checks grepped `zeam_0`/`ream_0`/`qlean_0`, so every health check and
the whole result summary reported nothing against the all-ethlambda
`validator-config.yaml` the local devnet uses now.
- Its scripts predate the Docker log caps that the newer skill scripts
set.

It was last touched functionally in #414; #532 only swept it for docs
consistency.

## Not carried over

The one capability with no replacement is `test-branch.sh`'s
build-tag-patch-restore step: build the current branch as
`ghcr.io/lambdaclass/ethlambda:<branch>`, point `ethlambda-cmd.sh` at
it, and restore the backup afterwards. `make docker-build` only produces
`:local`. Left out deliberately; worth adding to `devnet-runner` as a
small script if branch-image testing comes up again.

Author: MegaRedHand