feat: disable VS Code AI features by default

[AprilSylph]

Description

Because the little sparkle button on the commit message text input was bugging me.

This is committed via git add --force, which means local changes to .vscode (including re-enabling AI features in the workspace, if desired) remain untracked by default, so we won't randomly see changes to this file included in unrelated PRs.

Testing steps

1. Open this branch in VS Code
2. Go to the "Source Control" tab
   • Expected result: The commit message text input is unobstructed by useless buttons
   • Expected result: Probably some other stuff goes away too, but nothing of value is lost