Discarding changes doesn't work in Agents Window

1. discard changes as an option is only available under `Uncommitted Changes`, which isn't the default view. Since I work commit-by-commit, I prefer that to be the default and not. all branch changes
2. `Discard` seems like a no-op. Clicking it in the Changes list view, or in the diff view does nothing.

https://github.com/user-attachments/assets/d19a4e2b-5f51-4a97-aa2e-dc4fe6651b78

Start by reproducing the two behaviors in the Agents Window: the default changes view and the Discard action from both the Changes list and diff view. Trace the relevant Agents Window UI entry points after reproduction. Done means the preferred commit-by-commit view is default and Discard visibly removes the selected changes.

Written by the indexing model from the issue text.