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IntegrationRegistry::directAbilityNames() is removed (superseded by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/Abilities/AbilityRegistrar.php

@@ -18,7 +18,7 @@ final class AbilityRegistrar
 {
     public const CATEGORY = 'lit-mcp';
 
-    public function __construct(private IntegrationRegistry $registry)
+    public function __construct(private IntegrationRegistry $registry, private ?ExposurePolicy $policy = null)
     {
     }
 
@@ -38,7 +38,8 @@ final class AbilityRegistrar
 
     public function registerAbilities(): void
     {
-        foreach ($this->registry->abilities() as $ability) {
+        $policy = $this->policy ?? new ExposurePolicy();
+        foreach ($policy->enabled($this->registry->abilities()) as $ability) {
             wp_register_ability($ability->name(), $this->argsFor($ability));
         }
     }