refactor(agent): share the HID++ managers' retry pacing

The gesture, keyboard and host-switch managers each defined
`RETRY_DELAY = 1 s` and an identical `wait_for_deadline`. They live once in
`watchers::retry` now. It is a neutral module rather than part of the capture
managers' loop because the host-switch manager, whose loop is its own, uses
both.

Guard: agent-watcher-retry-owner flags a `RETRY_DELAY` constant or a
`wait_for_deadline` function defined elsewhere under watchers/ (6 hits on the
parent tree, 0 here).