Add poolmgrctl CLI client #59

There's currently no CLI for the gRPC API — operators/debuggers have to hand-write gRPC calls against PoolAdmin/Lease/Events.

Add a thin CLI client (e.g. poolmgrctl) supporting:

• Pool CRUD (create/update/delete/list) against PoolAdmin
• Listing active leases and manual claim/release against Lease
• Optionally, tailing the Events stream for a pool

This would live alongside the existing poolmgrd/poolmgr-hostagent binaries under cmd/.