Don't attempt to diagnose free-entry sections in --checkup

[rolandwalker]

Description

Sections such as [favorite_queries] are free-entry by the user, so a comparison to the distribution configuration per-item in those sections is not meaningful, and produces unhelpful output.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Review Summary
Change is small and aligns with PR intent. I don’t see correctness or security regressions in the diff. The main gap is test coverage for --checkup.

Findings (by severity)

1. Missing tests (medium)
   • There are no tests around do_config_checkup, and this PR changes its behavior. Please add a focused test that asserts entries under [favorite_queries], [init-commands], [alias_dsn], and [alias_dsn.init-commands] are not reported as “Unsupported” during --checkup. This would guard against regressions in future refactors.

Notes

• The skip list is inside the per-item loop, so it avoids item-level comparisons for free-entry sections as intended. This doesn’t affect section-level unsupported checks.

If you want, I can suggest a minimal test harness/fixture approach for --checkup given the current structure.