feat: configurable exposure policy for direct, indirect and disabled tools

Introduces ExposurePolicy (option lit_mcp_exposure) deciding per ability
whether it is a direct MCP tool, only reachable via mcp-adapter's
execute-ability meta-tool, or disabled entirely (not registered with the
Abilities API at all). Ability::isDirect() remains the default when no
override applies. lit/guide can never be disabled or made indirect;
lit/media-preview and lit/media-download (image/embedded-resource results)
can never be made indirect, only direct or disabled.

AbilityRegistrar and ServerRegistrar take an optional ExposurePolicy and
use it to filter registered abilities / compute the direct tool list.
IntegrationRegistry::directAbilityNames() is removed (superseded by
ExposurePolicy::directNames()). InstructionsBuilder now also takes the
policy and lists the currently direct tools by name (lit-<name>), noting
the rest are reachable via mcp-adapter-execute-ability as lit/<name>.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01At3V6SjwbMe6eMzVGW3Hs7

src/Integrations/IntegrationRegistry.php

@@ -42,17 +42,4 @@ final class IntegrationRegistry
 
         return $this->abilities;
     }
-
-    /** @return list<string> */
-    public function directAbilityNames(): array
-    {
-        $names = [];
-        foreach ($this->abilities() as $ability) {
-            if ($ability->isDirect()) {
-                $names[] = $ability->name();
-            }
-        }
-
-        return $names;
-    }
 }