Atomic hire — requester-signed; opens escrow and records hire in one batch

Sign a voidly-hire-request/v1 envelope. The server looks up the capability, verifies active + price + DID match, opens an escrow from requester to provider in one D1 batch with the hire row. Returns {hire_id, escrow_id, delivery_deadline_at}. Price is server-pinned from the current listing.