git_ui: Add starts_open setting to git panel (#51601)

Fixes #51542

Adds a `git_panel.starts_open` setting for parity with `project_panel`.
When set to `true`, the git panel opens automatically in new workspaces
that don't have saved panel state.

The default is `false`, matching current behavior (non-breaking).

### Usage

```json
{
  "git_panel": {
    "starts_open": true
  }
}
```

### Implementation

Follows the same pattern as `project_panel.starts_open`:

1. `GitPanelSettingsContent` - added `starts_open: Option<bool>` to the
settings schema
2. `GitPanelSettings` - added `starts_open: bool` field
3. `GitPanel` Panel impl - overrides `starts_open()` to read from
settings

Release Notes:

- Added `git_panel.starts_open` setting to control whether the git panel
opens automatically in new workspaces (default: false)

---------

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/src/reference/all-settings.md

@@ -5097,7 +5097,8 @@ See the [debugger page](../debugger.md) for more information about debugging sup
     "collapse_untracked_diff": false,
     "scrollbar": {
       "show": null
-    }
+    },
+    "starts_open": false
   }
 }
 ```
@@ -5112,6 +5113,7 @@ See the [debugger page](../debugger.md) for more information about debugging sup
 - `sort_by_path`: Whether to sort entries in the panel by path or by status (the default)
 - `collapse_untracked_diff`: Whether to collapse untracked files in the diff panel
 - `scrollbar`: When to show the scrollbar in the git panel
+- `starts_open`: Whether the git panel should open on startup
 
 ## Git Worktree Directory