feat(config): add validate command for sync config
Add `bd config validate` command to validate sync-related configuration:
- sync.mode: validates values (local, git-branch, external)
- conflict.strategy: validates values (lww, manual, ours, theirs)
- federation.sovereignty: validates values (none, isolated, federated)
- federation.remote: ensures set when sync.mode is 'external'
- Remote URL format: validates dolthub://, gs://, s3://, file://, etc.
Also validates existing config via doctor.CheckConfigValues (sync.branch,
routing.mode, etc.)
Closes: hq-ew1mbr.29
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
9ddd7a2620