ci(release): support prerelease self-host image tags

[JSONbored]

Parent: #1936

Problem

The release workflow and docs are close, but the current image release path is not quite beta/RC ready. The workflow accepts strict semver only, while the desired public posture is a prerelease image rather than a stable orb-v0.1.0 claim.

Requirements

• Support a prerelease self-host image tag such as orb-v0.1.0-rc.1 or orb-v0.1.0-beta.1.
• Mark the GitHub Release as prerelease for prerelease tags.
• Avoid moving latest for RC/beta tags unless we deliberately decide that is acceptable.
• Keep stable release behavior intact for future orb-vX.Y.Z tags.
• Preserve Sentry sourcemap upload and provenance/SBOM behavior for official releases.

Deliverables

• Update .github/workflows/release-selfhost.yml to accept prerelease tags safely.
• Define the image tag policy for rc, beta, stable, latest, and short SHA tags.
• Update release documentation to match the actual workflow behavior.
• Add or update tests/checks if the repo has workflow validation helpers.

Acceptance criteria

• Maintainer can trigger or tag a prerelease without workflow validation rejecting the version.
• The produced release is visibly prerelease.
• Stable release behavior remains unchanged.
• Docs explain exactly which tag external beta testers should pull.

Expected outcome

We can publish a beta-stable self-host image without overclaiming stability or confusing early operators.

[linear-code]

JSO-2022