fix(workflow): default updater_ref to v0 and validate ref before checkout

[d-lazenby]

Summary
This PR fixes a reusable workflow failure caused by an invalid default updater ref and aligns documentation with current release behaviour.

Closes #55

What Changed

• Updated workflow default updater_ref from v1 to v0 for both workflow_call and workflow_dispatch in .github/workflows/update-tf-modules.yml.
• Added an early validation step before source checkout to verify updater_ref exists as a branch or tag (or is SHA-like), with a clear actionable error message on failure in .github/workflows/update-tf-modules.yml.
• Updated docs/examples to use the current major channel v0 and corrected updater_ref default references.

Why

• The project is currently on 0.x releases, so v1 may not exist and causes low-signal checkout failures.
• Preflight validation provides clearer diagnostics and faster troubleshooting for callers.

Impact

• Existing callers that do not pass updater_ref now default to a valid major channel.
• Invalid refs now fail early with a clear message instead of a generic git fetch error.

[small-e]

Looks good to me