Automate the release process

[t-kalinowski]

Summary

Add a small, linear release command so maintainers do not need to coordinate version commits, CI, tagging, publication checks, artifact verification, and the development-version reset by hand.

User-facing changes

• Add scripts/release.sh VERSION for stable releases from a clean, up-to-date main checkout on supported macOS and Linux hosts.
• Run the complete local check, push the stable version, watch its exact CI run, create the annotated tag, and watch the tag-driven release workflow.
• Download and checksum the current host's archive, smoke-test ir, rx, and a short R execution, then restore VERSION+dev and watch its CI run.
• Document prerequisites, the release sequence, and manual recovery after a failure in RELEASE.md.

Internal changes

• Keep the script to a single fail-fast path with no automatic recovery state machine.
• Require both origin fetch and push URLs to resolve to r-lib/ir before changing files.
• Add two black-box tests using real local Git repositories and a small GitHub shim, covering invalid input and the complete release sequence.

Notes

The complete local check passes with 198 tests. The release script also passes Bash syntax and ShellCheck validation.