feat: add variant field to disambiguate nodes command JSON output

[flyingrobots]

Both nodes --id and nodes (list) emit "command": "nodes", requiring consumers to sniff the payload shape (id field vs nodes array) to select the correct schema. A variant field (e.g. "variant": "detail" vs "variant": "list") would let routers select the right schema without shape inspection.

Also applies to review --json (list) vs review --batch --json (batch) and export --json (data) vs export --file --json (file).

Raised during IRONCLAD M9.T1 (#205, PR #217).