fix(cli): emit consensus_threshold=0 in vote list JSON when whitelist is empty

[ebios-star]

Summary

Closes #753.

`gitt vote list --json` (cli/issue_commands/vote.py:248-249) computed `required = (n // 2) + 1` unconditionally, so an empty validator whitelist produced `{"count": 0, "consensus_threshold": 1}` — a 1-of-0 threshold that no integer vote count can satisfy.

JSON consumers (dashboards, monitoring scripts) could not detect the empty-whitelist edge case from this field alone, even though the human-mode branch already prints "No validators whitelisted." and skips the threshold line entirely.

Now: `consensus_threshold == 0` when no validators are whitelisted; the existing `(n // 2) + 1` majority is preserved for non-empty whitelists.

Related Issues

Closes #753.

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Other

Testing

• New `tests/cli/test_vote_list_consensus.py` covers three cases:
  • empty whitelist → `consensus_threshold == 0`
  • 3 validators → `consensus_threshold == 2`
  • 1 validator → `consensus_threshold == 1`
• `uv run --extra dev pytest tests/` — 446 tests pass (443 baseline + 3 new).

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[ebios-star]

@anderdc @LandynDev — friendly ping for review when you have a moment.

Closes #753. When the validator whitelist is empty, gitt vote list --json previously emitted {"count": 0, "consensus_threshold": 1} — a 1-of-0 threshold that no integer vote count can satisfy. JSON consumers (dashboards, monitoring) now see consensus_threshold: 0 for empty whitelists. Existing (n // 2) + 1 majority preserved for non-empty cases. 3 new tests, full test suite passes.

Happy to rebase if needed.