feat(multisig): add multisig action provider for external coordination

[aetos53t]

Summary

This PR adds a MultisigActionProvider enabling AgentKit agents to participate in multi-agent multisig wallets (2-of-3, 3-of-5, etc.) with external coordination protocols.

Problem

Currently, AgentKit agents can only manage their own individual wallets. There's no way for multiple agents to jointly control funds via multisig.

Solution

Three new actions for external multisig coordination:

Action	Description
sign_digest	Sign a raw 32-byte digest for Taproot/EVM coordination
sign_safe_transaction	Sign a Safe (Gnosis Safe) transaction hash
get_multisig_pubkey	Get public key for registering with coordinators

Use Cases

• Multi-agent treasuries: 2-of-3, 3-of-5, etc.
• Cross-provider coordination: AgentKit + aibtc + Claw Cash agents sharing one multisig
• Safe multisig participation: Sign transactions for existing Safes
• DAO treasuries: Multiple AI agents managing shared funds

Architecture

┌──────────┐    ┌──────────┐    ┌──────────┐
│ AgentKit │    │   aibtc  │    │Claw Cash │
│  Agent   │    │  Agent   │    │  Agent   │
└────┬─────┘    └────┬─────┘    └────┬─────┘
     │               │               │
     └───────────────┼───────────────┘
                     │
          ┌──────────▼──────────┐
          │   Coordination API  │
          │  (Agent Multisig)   │
          └──────────┬──────────┘
                     │
              ┌──────▼──────┐
              │   2-of-3    │
              │  Multisig   │
              └─────────────┘

Security

• Non-custodial: Private keys stay with agents
• No blind signing: Documentation emphasizes validating transactions before signing
• Standard protocols: Uses existing sign() method, BIP-340/EIP-712 compliant

Testing

• Full unit test coverage for all actions
• Input validation tests (length, format, normalization)
• Error handling tests

Related Work

• Coordination API: https://github.com/aetos53t/agent-multisig-api (mainnet proven on Bitcoin)
• aibtc integration: feat(signing): add schnorr_sign_digest and schnorr_verify_digest for Taproot multisig aibtcdev/aibtc-mcp-server#163 (merged)
• Issue feat: Multi-agent wallet coordination for shared treasury management #960: feat: Multi-agent wallet coordination for shared treasury management #960

Checklist

• Unit tests added
• Documentation (README.md) included
• Follows existing action provider patterns
• Security considerations documented

[cb-heimdall]

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