feat: group tools by integration with inherited group mode and "nur lesen" switch

Tools tab shows one block per integration with a group mode (inherited by its
tools) and a read-only switch that disables all non-readonly abilities of the
group. ExposurePolicy resolves tool override > group override > tool default;
lit/guide keeps its default, direct-only tools fall back to direct under a
group "indirekt". Group entries live in lit_mcp_exposure under "@group:<id>"
and "@readonly:<id>"; the old flat format stays valid. The ability->group map
comes lazily from IntegrationRegistry (Divi is not loaded at plugins_loaded).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/Plugin.php

@@ -71,7 +71,7 @@ final class Plugin
             new DiviIntegration($schema, $changeLog)
         );
 
-        $policy = ExposurePolicy::fromOption();
+        $policy = ExposurePolicy::fromOption()->withGroups($registry->groupMap(...));
         (new AbilityRegistrar($registry, $policy))->register();
         (new ServerRegistrar(
             $registry,