This makes context essentially work the same way as `read-file`,
increasing the likelihood of cache hits.

Just like with `read-file`, we'll notify the model when the user makes
an edit to one of the tracked files. In the future, we want to send a
diff instead of just a list of files, but that's an orthogonal change.


Release Notes:
- agent: Improved caching of files in context

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Co-authored-by: Antonio Scandurra <me@as-cii.com>