Allow excluding paths from Chat changed files pane and review workflow

## Feature request

When I use Chat to make changes, I want the changed files pane to show only the files I need to review.

Some agent workflows intentionally create local, untracked artifacts that are useful during development but are not part of the code or documentation review. For example:

- `.superpowers/**`
- `docs/superpowers/**`

I can add these to `.gitignore` and hide them from VS Code's Explorer, but their edits still appear in Chat’s changed files pane alongside the files I asked Chat to change.

## Current behavior

1. Configure Git and Explorer to ignore or hide artifact paths such as `.superpowers/**` and `docs/superpowers/**`.
2. Use Chat to edit a tracked file.
3. During the same workflow, Chat or its supporting process creates or updates files in one of those artifact paths.
4. View the Chat changed files pane.

The UI includes both the tracked file I need to review and the modified Superpowers artifacts.

## Requested behavior

Provide a Chat-specific setting that omits matching files from the Chat changed files pane, change count, and bulk review/apply UI.

## Why existing exclusions do not solve this

- `files.exclude` controls Explorer visibility.
- `search.exclude` controls visibility in search results.
- `.gitignore` controls Git tracking.

None of these controls which files appear in Chat’s changed files pane.

## Related issue

[#317592 — Git modified files show in Copilot Chat window](https://github.com/microsoft/vscode/issues/317592) reports the broader problem of unrelated Git changes appearing in Chat. This request is narrower: it asks for an intentional, path-based filter for known artifact files.

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Drafted with AI, reviewed and edited by me.