refactor(agent): call the host-switch session stamp an epoch

The host-switch manager tagged each spawned session with a `generation: u64`
so a stale completion cannot settle its successor. On this codebase's IPC
surface "generation" is the agent's observe stamp; the same idea two modules
over is already `CaptureEpoch` and `same_epoch` (runtime/button.rs).

It is `SessionEpoch` now, a newtype so the stamp cannot be confused with the
host numbers beside it. `next_generation` held the last stamp issued, not the
next one, and is `last_epoch`.