AI agents on blockchains need wallets that work without human intervention—yet no single component should be trusted with the full private key. Existing approaches force a choice: hot wallets grant autonomy but no constraints; smart accounts enforce on-chain policies but with gas overhead and scoped session keys; and multi-party computation (MPC) wallets treat all co-signers as interchangeable. We present Guardian Wallet, a role-gated MPC wallet where the private key is split into three shares, each bound to a distinct operational role: the agent (autonomous execution), the governor (policy enforcement), and the sovereign (human oversight). Any two of the three shares can co-sign a transaction, but each pair carries different authority: normal operation, manual override, or emergency bypass. The governor only releases its share after policy checks pass, making the wallet role-gated: which pair co-signs determines what is allowed. We validate this design on Ethereum using the CGGMP threshold ECDSA protocol. Key generation completes in 1 s with pre-computed auxiliary data, signing takes 1.1 s median on localhost, and policy evaluation adds under 2 ms. We compare against smart accounts, hosted MPC wallets, and decentralised MPC using verified production deployments.