Add manual snapshot releases and remove temporary diagnostics

[gonzaloriestra]

WHY are these changes introduced?

/snapit fails during npm OIDC token exchange, while manual release diagnostics succeed. A manual snapshot release lets us test publishing a development branch without an issue_comment trigger.

WHAT is this pull request doing?

Add snapshot to the Release workflow's Tag menu and support it in the existing release script. In Run workflow, select a branch containing this change and choose snapshot to publish. This uses the manual release path, including the unscoped shopify package.

Remove the temporary OIDC diagnostics, /snapit-oidc trigger, diagnostics-only input, and verbose npm logging added in #8581 and #8582.

Validated workflow YAML, Bash syntax, and release routing for all supported tags with build and publish commands stubbed. Confirmed the workflow matches its pre-diagnostics version apart from the manual snapshot option. No packages were published during validation.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add