Make the gridscan frequency configurable and set to 500Hz as the default

Currently the gridscan frequency is hard-coded in `agamemnon.py` - this should be a configurable parameter, we should default to 500Hz which is the current production value.

## Acceptance Criteria
- Specific criteria that will be used to judge if the issue is fixed

Start in agamemnon.py, locate the hard-coded gridscan frequency, and read how nearby parameters are defined and passed. Make the frequency configurable with 500Hz as its default, then verify that the existing gridscan path uses the default and accepts an override.

Written by the indexing model from the issue text.