BudgetLedger interface + InMemoryBudgetLedger reference impl (reserve/settle/
release with refund of unused holds; mirrors the b1dz budget+ledger concept).
withBudget() wraps any SwarmRunner: reserves up front (throws SwarmError 402 when
out of funds, before running), settles actual cost on success, releases on error.
AgentIdentity DID aligns with @logicsrc/agentstack.
8 new tests; 27/27 pass.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
