feat(snap): publish prerelease builds of the snap to the latest/beta channel #4869

User Story

I want to be able to install pre-release builds of OpenShell via the snap package, as is possible for other packaging formats.

Problem Statement

The pre-release builds are not currently published as a snap.

Impact / Why This Matters

Users of the OpenShell snap package may wish to test prerelease builds, but don't have a convenient way to do so without installing .snap artifacts directly from CI, which lack store assertions for interface connections and aliases.

Proposed Design

Publish the pre-release .snap artifacts to the latest/beta channel.

Acceptance Criteria

• The OpenShell pre-release .snap artifacts are published to the latest/beta channel of the openshell snap

Alternatives Considered

Considered a more complicated system of publishing the latest/beta immediately when artifacts are created, then promoting the artifact into latest/candidate after qualification, but this adds a lot of complexity, especially when releases are usually published to stable anyway shortly after qualification finishes.

Agent Investigation

N/A

Checklist

• I've reviewed existing issues and the published docs
• This is a design proposal, not a "please build this" request