fix: install dependencies before beta versioning

[heimanba]

Summary

• install the locked workspace dependencies before invoking the Changesets CLI
• enter prerelease mode only when the beta release branch does not already exist

Behavior / risk

The Prepare Beta Release workflow currently resolves an unrelated changeset package and fails before versioning because dependencies are not installed. This preserves the existing beta branch/version rules while ensuring the repository-pinned Changesets CLI is used.

Validation

• reproduced the failure in Prepare Beta Release run 29735313780
• bun run changeset -- --help
• git diff --check
• push verification profile: typecheck, architecture checks, tests, and changed-file lint