Feature Request: local "global" context

At the moment, we can create a global context via `cC`, but this is truly global and applies to AmazonQ on all projects.

This feature request is about allowing a local "global" context that users can add to a local git repo, for example.

For example, the following md would be added to all prompts when AmazonQ is started within `$PWD/repo` but not to other repos:

```
$PWD/repo/.amazonq/global-context.md
```

Start by tracing the existing `cC` global-context behavior and how prompts are assembled when AmazonQ starts. Determine how a repository-local `.amazonq/global-context.md` would be discovered only within that repository, then verify that its contents affect prompts there but not in other repositories.

Written by the indexing model from the issue text.