Add release-plz automation

[robjtede]

Summary

• add release-plz automation for the five serde-utils workspace crates
• create draft release PRs and draft GitHub releases using the repository's existing crate-specific tag convention
• consume the shared x52-release-tools package through the default Nix devshell
• enter that devshell in the release workflow with the pinned nicknovitski/nix-develop action
• pass release-plz JSON through environment variables for changelog and release-note processing

Impact

Pushes to main can now create draft release PRs, publish crates after those PRs merge, and update crate changelogs and GitHub release notes using the shared x52 tooling.

The workflow installs Nix and enters the devshell only when release-plz created a pull request or release. The repository's existing crate-vX.Y.Z tags and per-crate changelogs are preserved.

External prerequisite

Before merging, configure crates.io Trusted Publishing for detrim, double-int, serde-bool, serde-decimal, and serde-secrecy with repository x52dev/serde-utils and workflow release.yml.

The repository's release label has been created for the configured release PR metadata.

Validation

• nix develop -c just fmt
• nix develop -c just check
• nix develop -c just test (82 tests and 7 doctests passed; warning-denied docs passed)
• nix flake check
• verified x52-bump-changelogs and x52-update-release-notes are present in devShells.default