fix(cli): correct Relation attribute names in list-relations command

[Copilot]

PR #349 introduced 8 new read/list CLI commands but the list-relations command referenced attributes that don't exist on the Relation model (relation_type, from_id, to_id), causing mypy to fail in CI.

Changes

• src/vouch/cli.py: Added 8 CLI commands with correct Relation field names:
  • read-{claim,page,entity,relation} — fetch a single approved artifact by id
  • list-{claims,pages,entities,relations} — enumerate all approved artifacts of a type
  • list-relations now uses relation.source, relation.relation, relation.target to match the model definition

# Before (broken — these fields don't exist on Relation)
rel_type = relation.relation_type or "unknown"
click.echo(f"{relation.id:50} {relation.from_id} -> {rel_type} -> {relation.to_id}")

# After (correct field names from models.Relation)
output = f"{relation.id:50} {relation.source} -> {relation.relation} -> "
output += relation.target
click.echo(output)