feat(cli): harden agent loan and subscription flows

[StevenACZ]

Summary

• fix loan payments to send payAmount and add a new loans mark-paid command
• add strict numeric parsing, post-payment verification, and real --clear-* flags
• enrich subscriptions list output with derived billing context for AI agents

Verification

• bun run format
• bun run typecheck
• bun run lint
• bun run test
• bun run build