Automatic Eviction of Terminal MultiSig Operations

[connelevalsam]

closes #137

Summary
Implemented automatic eviction of terminal multi-sig operations.
src/types/multisig.ts — Added optional terminalAt?: number to MultiSigOperation, recording when an operation reaches a terminal status (submitted/expired).
src/escrow/multisig.ts

• Added DEFAULT_MULTISIG_RETENTION_MS (5 min) and optional retentionMs constructor option (backward compatible).
• Added MultiSigEscrowClientOptions interface.
• Added public prune() method that evicts terminal-status operations older than the retention window — the operations Map can no longer grow unboundedly.
• listOperations now calls prune() first, so it returns only retained (non-evicted) operations (documented in typedoc).
• _markTerminal helper sets terminalAt whenever an operation is flipped to expired or submitted in addSignature, getMultiSigStatus, and submitWhenReady.

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