Add a setting to disable the integrated Git worktrees UI (Source Control listing + branch checkout prompts)

- VS Code Version: 1.122.1 (commit 8761a5560cfd65fdd19ce7e2bd18dab5c0a4d84e, x64)
- OS Version: Linux x64 (Arch, kernel 7.0.10)

### The request

Please add a setting to fully opt out of the integrated Git worktrees UI. Concretely, a way to disable:

1. The **worktrees listing** in the Source Control view, and
2. The **worktree-aware prompts** on branch checkout.

Some of us deliberately keep each worktree as an independent, self-contained window and never want cross-worktree UI. Today there is no setting that turns this off. This setting was still available until a recent update.

### Steps to Reproduce

1. Create a repo with two linked worktrees, e.g. `repo/` (primary) and `repo-secondary/` (a `git worktree` of it on a different branch).
2. Set `"git.detectWorktrees": false` (and `"git.detectWorktreesLimit": 0`).
3. Open only `repo/` in a window.

Observed:
- The other worktree still shows up in the Source Control view (worktrees are listed regardless of `git.detectWorktrees`).
- Switching to a branch that is checked out in the other worktree triggers a worktree prompt/conflict dialog instead of just performing (or refusing) the checkout in the current worktree.

Expected (with an opt-out setting enabled):
- No worktree listing in Source Control for the current window.
- No worktree prompts; branch operations act only on the worktree of the current window.

### Suggestion

A single opt-out, e.g. `"git.worktrees.enabled": false` (or `"git.showWorktrees": false`), that suppresses the worktrees artifact group in Source Control and skips the worktree-aware branch prompts, so each worktree window behaves as a standalone repository.

Related: #278774 (closed; only flipped the default), #298496 (about the Copilot agent creating worktrees, not the SCM UI).