Release workflow: always publish as a pre-release, promote to latest by hand

[drowaudio]

Every tag build now publishes its release as a pre-release, whatever the tag. Previously only tags containing a hyphen (v2.0.0-beta1) were pre-releases, so tagging v2.0.0 would have made it "Latest" immediately, and every script downloading releases/latest would have moved from 1.0.4 to 2.0.0 at once.

Releases are now promoted by hand once checked:

gh release edit vX.Y.Z -R Tracktion/pluginval --prerelease=false --latest

What

• build.yaml: prerelease: true on the release step.
• CLAUDE.md: the release process says the release is published as a pre-release and how to promote it.

Effect on the GitHub Actions

None: they download pluginval by exact tag, which works for pre-releases (they already use the v2.0.0-beta3 pre-release). The plan is to switch their default pluginval-version to latest when 2.0.0 is promoted, so promoting a release is also what moves the actions on, and the per-release action.yml bump goes away.

Testing

Only runs on a tag push, so it'll be exercised by the next tag. The YAML parses, and the step's prerelease value is true.

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