refactor(agent): spell repeating durations as _PERIOD

Eight constants that set how often something repeats were spelled three other
ways: `SHUTDOWN_POLL_INTERVAL`, `IDLE_RECOVERY_INTERVAL`,
`OBSERVER_RETRY_INTERVAL`, `RECOVERY_SCAN_INTERVAL` and `FRAME_INTERVAL` in
agent-core, `IDLE_TICK` beside them, and `PAIRING_POLL_TICK` and
`OBSERVE_TICK` in the mock agent. The codebase's word for a repeating duration
is `_PERIOD`, which leaves `_TIMEOUT` for a duration that bounds one wait.

`IDLE_TICK` becomes `IDLE_CHECK_PERIOD` and `OBSERVE_TICK` becomes
`OBSERVE_POLL_PERIOD`, since neither named what repeats.
`MISSING_TICKS_UNTIL_GONE` is a count, not a duration, and keeps its name.