Add /-/debug hub page with debug_menu() plugin hook

Replace the scattered debug tool links in the app menu with a single
"Debug" link to a new /-/debug page. This page aggregates all debug
tools using a new debug_menu() plugin hook, which plugins can implement
to contribute DebugItem(title, description, path) entries.

Hook implementations are responsible for their own permission checks,
so the page only shows items the current actor can access. Core debug
items (databases, plugins, versions, settings, permissions, etc.) are
registered via default_debug_menu.py.

https://claude.ai/code/session_01QE3BkTNRLvLEpLXy7ZDCeU

tests/plugins/my_plugin.py

@@ -2,7 +2,7 @@ import asyncio
 from datasette import hookimpl
 from datasette.facets import Facet
 from datasette import tracer
-from datasette.permissions import Action
+from datasette.permissions import Action, DebugItem
 from datasette.resources import DatabaseResource
 from datasette.utils import path_with_added_args
 from datasette.utils.asgi import asgi_send_json, Response
@@ -354,6 +354,18 @@ def forbidden(datasette, request, message):
         return Response.redirect("/login?message=" + message)
 
 
+@hookimpl
+def debug_menu(datasette, actor, request):
+    if actor:
+        return [
+            DebugItem(
+                title="Test debug item",
+                description="From test plugin",
+                path="/-/plugins",
+            )
+        ]
+
+
 @hookimpl
 def menu_links(datasette, actor, request):
     if actor: