CLI: improve GitHub API failure guidance for audit errors

[SergioChan]

Summary

• add a targeted fatal help block when an audit fails due to GitHub API access/rate-limit style errors (for example HTTP 403 or 'couldn't list tags')
• keep the existing underlying error chain, but provide immediate actionable guidance to reduce confusion
• suggest token configuration and offline mode as recovery paths

Testing

• Not run locally in this environment because the Rust toolchain is unavailable (cargo: command not found), so I could not execute the test suite.
• Change is intentionally minimal and isolated to error-report rendering in main.rs for Error::Audit.

Related

Fixes #1252

[woodruffw]

Please don't do this.

Multiple things here:

1. You haven't filled out the required template for PRs. This template is mandatory.
2. You haven't engaged at all with the discussion in #1252, or the current PR in #1609.
3. Your PR isn't at all general -- it doesn't handle rate limiting outside of audits, or properly distinguish between GitHub origins and other origins.