Can an AI agent post Bitcoin as collateral without giving up the keys?

A recent debate in the agent-economy timeline centered around settlement and collateral. Agents must decide who can move collateral after it's posted. For Bitcoin, this is a problem due to its limitations on contract logic. Wrapping Bitcoin with a custodian or bridge contract is a common solution, but it introduces new risks, such as converting a bearer asset into a claim and making collateral integrity dependent on the solvency of the custodian. A collateral vault takes the opposite approach, keeping Bitcoin native and only transferring information across chains.