refactor(agent): name what HOLD and PERIOD hold and repeat

`pairing.rs` had a bare `HOLD` and `binary_watch.rs` a bare `PERIOD`. Both read
fine beside their one use and say nothing in a grep or a stack of constants:
they are `PAIRING_HOLD`, the name the mock agent already gives its copy of the
same long-poll window, and `BINARY_CHECK_PERIOD`.