fund command - domain budget management

New CLI command for managing agent domains and budgets. Separate from agentd (which stays a pure process supervisor).

Commands:

• fund create <domain> --budget <cents> (create a domain with a budget)
• fund add <domain> <cents> (inject more budget)
• fund status (show all domains, budget vs spend)
• fund spend <domain> (show spend breakdown)

Data model:

• domains table: name, budget_cents, spent_cents, revenue_cents
• Reads agentd cost_cents data to compute per-domain spend
• agentd runs get tagged with a domain

Later: revenue-linked auto-replenishment (e.g., PIL Stripe revenue credits its domain).

This is part of the agent economy design: ownership + budget as lightweight coordination.