Add device pair - #30

Adds the device pairing command to the management CLI.

• accept device pair <fleet_id> [name]
• make the device name optional without changing positional compatibility
• show pair first in the Devices help list
• wait long enough for the server and firmware confirmation round trip
• keep user-facing errors and confirmation output at the device/fleet level

Verification:

• CGO_ENABLED=0 gofmt -l .
• CGO_ENABLED=0 go mod tidy left go.mod and go.sum unchanged
• CGO_ENABLED=0 go vet ./...
• CGO_ENABLED=0 go test ./...
• CGO_ENABLED=0 go build ./...
• git diff --check
• the named command paired a bench device through a compatible local server and firmware and returned success after device acknowledgement