Improve release script

[JeanChristopheMorinPerso]

This PR changes the release script to better reflect my current workflow. It is more opinionated and also will try to prevent as many mistakes as possible.

I changed the interface to be more user friendly. It makes use of subcommands:

• release-rez.py changelog generates the changelog based on upstream commits. It gets the commits between the previous release and HEAD and for each commits gets the corresponding PR. The output is the same as before.
• release-rez.py tag same as before.
• release-rez.py release creates the release. It now properly formats the release notes to be compatible with what GH expects. It also now prints a preview of the release and asks you if you want to actually create the release. This gives us an opportunity to catch any mistakes before it's too late.

The push step is gone. It was not useful IMO.

In terms of robustness, the checks for remotes was augmented to do this:

• If there is more than one remote, print an error and exit.
• If there is only one remote but its name is not origin, print an error and exit.

This removes a few footguns with the previous version of the script.

Lastly, I updated the release instructions to match the new script.

I believe that with these changes in place, the release process has been simplified and most importantly, the footguns have been removed. This will hopefully lead to streamlined releases without the additional stress caused by the previous script.

I used some help from an LLM to create this PR. Mostly for the regexes and the GH API calls. Corresponding threads:

• https://ampcode.com/threads/T-019e800c-848c-71be-b822-39e3095945d9
• https://ampcode.com/threads/T-019e803f-4399-76fc-85bf-3400699ccdf0

[Copilot]

Pull request overview

This PR modernizes the Rez release workflow by replacing the release script’s step flags with subcommands and updating the release documentation to match the new process.

Changes:

• Adds changelog, tag, and release subcommands to release-rez.py.
• Adds automatic PR discovery and GitHub release creation preview/confirmation.
• Updates RELEASE.md with the revised release procedure.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
release-rez.py	Reworks release automation, changelog generation, tagging, and GitHub release creation.
RELEASE.md	Documents the new subcommand-based release workflow.

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