feat: workflow improvements for immutable releases

[mosteo]

Github requires a release to be not a draft to trigger actions. This can be a quagmire if some build fails, as release watchers will have been notified, immutable releases don't allow uploading assets for manual fixing, etc.

With this patch, we instead trigger release actions by pushing a tag. Once builds succeed, the release is automatically created in draft mode, which doesn't yet notify anybody and allows fixing the release if something goes wrong. We manually publish the draft once everything is OK.

Tested in https://github.com/mosteo/alire/actions/runs/25383957370

[Copilot]

Pull request overview

This PR adjusts Alire’s release automation to avoid relying on GitHub’s “published release” event (which requires leaving draft mode), by triggering release builds from a pushed semantic-version tag and creating a draft release only after successful builds—reducing the risk of notifying watchers or getting stuck with immutable release assets.

Changes:

• Update the release CI workflow to trigger on push of vX.Y.Z tags instead of release: published.
• Create a draft GitHub Release automatically (on successful tag CI), then upload built assets to that draft.
• Update RELEASING.md to document the new tag-push + draft-release publishing process.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
RELEASING.md	Documents the updated release procedure: push tag → draft release created → edit → publish.
.github/workflows/ci-release.yml	Switches release triggering to tag pushes and creates a draft release prior to uploading release assets.