Add flags argument filter

## Prerequisites

- [ ] This issue is an EPIC issue (add label: EPIC).
- [ ] This issue is an EPIC TASK (add issue to EPIC description).

Select one OR another:

- [x] I'll create a PR to implement this feature (assign to yourself).
- [ ] Someone else should implement this (describe it well).

## Feature description

Since argument parsing happens after argument filtering, it is currently practically impossible to filter on flag values. As such a specialized filter is required for such arguments.

## Additional Information (feature drawings, files, logs, etc)

The issue does not name files or tests; start by locating the argument-filtering and argument-parsing entry points in the Go code. Define the filter behavior for flag values, then add coverage showing that filtering works when argument parsing occurs afterward.

Written by the indexing model from the issue text.