Agents Window: Filter or collapse test files in the Changes multi-diff editor

When reviewing an agent session's changes, test files can dominate the Changes multi-diff editor and make it harder to focus on product-code changes.

Add a way to de-emphasize files matching configurable filename/path patterns in the Agents Window Changes multi-diff editor.

Possible behavior:
- Allow users to configure glob patterns for test/generated/other low-priority files.
- Either hide matching files from the multi-diff editor with a clear way to reveal them, or keep them present but initially collapsed.
- Clearly indicate when files are filtered or auto-collapsed and how many are affected.
- Keep the underlying changes intact and accessible; filtering must never discard changes or affect commit/review operations.
- Preserve explicit user expansion/visibility choices where practical.

The primary goal is to let reviewers focus on product-code changes while retaining quick access to test changes.