Wire AgentRegistry + AgentEscrow to MPC committee custody via luxfi/mpc

[abhicris]

Problem

AgentEscrow.sol today releases on payee signature (ECDSA recover) or after timeout + challenge_period. There is no notion of a multi-party custody committee — every agent operates a single-key wallet, which is the single largest footgun in autonomous on-chain agents.

Closed issue #1 (MPC key management) was opened pre-Lux-collab and stalled. Reopen the surface with a concrete shape that maps onto luxfi/mpc + luxfi/threshold so committee membership is real, not aspirational.

Proposed Approach

Two surfaces:

On-chain (AgentRegistry.sol extension):

• registerAgentCommittee(bytes32 agentId, bytes32 committeeRoot, uint8 threshold, uint8 size)
• committeeRoot is the Merkle root of committee member public keys (one of them MUST be a luxfi/mpc committee identifier).
• AgentEscrow gates releasePayment on a successful verifyCommitteeSig(committeeRoot, threshold, signature, message) precompile call (proposed at 0x0e or via luxfi/mpc precompile if it exists).
• Rotation: rotateCommittee(bytes32 agentId, bytes32 newRoot) is gated on a 2-of-N quorum of the current committee.

Off-chain (switchboard/mpc_wallet.py):

• New module wraps the luxfi/mpc client. Agent calls MpcWallet.sign(transcript) which routes to committee members, collects partial signatures, aggregates into a single Schnorr / FROST-style signature acceptable to the on-chain verifier.
• Per-agent committee config in ~/.switchboard/agents/<agent_id>/committee.toml.
• One reference committee composition: kcolbchain_node, hanzo_node, lux_mpc_node (3-of-3 for v1).

Acceptance Criteria

• AgentRegistry.sol extended with committee registration + rotation + verification call
• switchboard/mpc_wallet.py shipped with luxfi/mpc client integration
• Test vector: 3-of-3 committee signs a releasePayment call, on-chain verifier accepts
• Doc: docs/mpc-committee-custody.md covering threat model, rotation, slashing-on-double-sign
• Issue Implement agent wallet with MPC key management #1 cross-linked and closed as superseded once this lands

References

• Issue Implement agent wallet with MPC key management #1 — Implement agent wallet with MPC key management (CLOSED, superseded by this)
• internal: see partner integration map section "Lux" — MPC custody is the wedge for the Lux/Hanzo committee composition
• luxfi/mpc + luxfi/threshold