Adds a `usage` subcommand that pings `/v1/usage` with the stored token and
displays balance, plan, 30-day explore/token counts, and allowance reset date.
Degrades quietly in all non-happy-path states — signed out, BYO endpoint, or
unreachable server — so managed reasoning remaining optional doesn't change.
Also extends `OffloadUsage` with the fields the endpoint already returns
(`unlimited`, `banned`, `tokensLast30`, `callsLast30`, `creditsLast30`) that
were previously untyped.