The `deploy-shim` job downloaded an artifact named `wp`, but the build
job uploads it as `wp-cli-phar` containing `wp-cli.phar`, so the download
failed and the subsequent `chmod`/`git add` had no file to operate on.

Download the correct artifact and rename the Phar to `wp`, which is what
`wp-cli/wp-cli-shim` declares as its `bin`.

Also align the job with the existing `deploy` job:

* Give it a distinct display name, as both were labelled "Deployment".
* Pin `actions/checkout`, `actions/download-artifact` and
  `ad-m/github-push-action` to the same SHAs used elsewhere in this
  workflow, instead of floating tags and `@master`.
* Push to `main` explicitly. Without it the action defaults to the
  current ref, which would push to a branch named after the bundle's
  ref rather than the shim's default branch.
* Push tags, so the "Tag stable release" step is no longer a no-op.
* Skip on `workflow_dispatch`, matching `deploy`.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RHtjyXkZh8X16sBgmidSQi