Workflows: replace actions/checkout with plain git

The runner failed resolving/running the external checkout action; a
git fetch of GITHUB_SHA with the workflow token needs neither action
resolution nor a node runtime in the job container, and works against
the private repo. Also document that a bare `cargo release` tags the
current version (the path for 0.1.0, which is already the workspace
version).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -75,7 +75,8 @@ with `--no-default-features` for systems without it.
 ## Releasing
 
 ```console
-$ cargo release patch      # or minor / major — see release.toml
+$ cargo release            # tag + push the current version as-is
+$ cargo release patch      # or minor / major to bump first — release.toml
 ```
 
 This bumps the workspace version, tags `vX.Y.Z`, and pushes; the tag