CEXT-6421: Publish previews in npm

[obarcelonap]

Description

Preview (alpha, per-PR /snapshot) and internal (beta) package snapshots now publish to the public npm registry instead of Artifactory, reusing the same npm token and publishConfig that stable releases already use. Also drops the now-unused release-channel branching in the Slack announcement, defaults the npm registry URL in the shared setup action, and fixes a leftover pre-release notice that incorrectly said beta snapshots weren't publicly available.

Related Issue

N/A

Motivation and Context

Artifactory-hosted snapshots caused two recurring problems:

• Consumers testing a preview from another public GitHub repo couldn't install alpha/beta packages at all, since Artifactory isn't reachable without internal credentials.
• Anyone who did have access needed to keep tweaking their local .npmrc to point at Artifactory, which was a poor developer experience for something as routine as trying out a preview build.

Publishing snapshots to the public npm registry removes both frictions, at the cost of alpha/beta versions being publicly visible (mitigated via the existing deprecate workflow if one needs to be pulled back).

How Has This Been Tested?

• pnpm test for the affected scripts package (40/40 passing).
• pnpm typecheck across the whole monorepo.
• pnpm lint (biome) on all changed files.
• Manual review of the updated workflow YAML for the preview, internal, and public publish flows.

Screenshots (if appropriate):

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have signed the Adobe Open Source CLA.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have read the DEVELOPMENT document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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Latest commit: fe8c819

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