feat: emit init-time notice for git extension default change (#2165) (#2432)

Add a non-blocking Panel notice during `specify init` when the git
extension auto-enables, informing users that starting in v0.10.0 this
will require explicit opt-in via `specify extension add git`.

- src/specify_cli/__init__.py: track successful git extension install
  and display yellow "Notice: Git Default Changing" panel
- tests/integrations/test_cli.py: integration test validating notice
  content (v0.10.0 timeline, opt-in messaging, migration command)
- docs/reference/core.md: user-facing NOTE about the upcoming change

Closes #2165

src/specify_cli/__init__.py

@@ -1264,6 +1264,8 @@ def init(
     ]:
         tracker.add(key, label)
 
+    git_default_notice = False
+
     with Live(tracker.render(), console=console, refresh_per_second=8, transient=True) as live:
         tracker.attach_refresh(lambda: live.update(tracker.render()))
         try:
@@ -1360,6 +1362,7 @@ def init(
                             manager.install_from_directory(
                                 bundled_path, get_speckit_version()
                             )
+                            git_default_notice = True
                             git_messages.append("extension installed")
                     else:
                         git_has_error = True
@@ -1530,6 +1533,18 @@ def init(
         console.print()
         console.print(deprecation_notice)
 
+    if git_default_notice:
+        default_change_notice = Panel(
+            "The git extension is currently enabled by default during [bold]specify init[/bold].\n"
+            "Starting in [bold]v0.10.0[/bold], this will require explicit opt-in.\n"
+            "Use [bold]specify extension add git[/bold] after init when needed.",
+            title="[yellow]Notice: Git Default Changing[/yellow]",
+            border_style="yellow",
+            padding=(1, 2),
+        )
+        console.print()
+        console.print(default_change_notice)
+
     steps_lines = []
     if not here:
         steps_lines.append(f"1. Go to the project folder: [cyan]cd {project_name}[/cyan]")