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IntegrationRegistry::directAbilityNames() is removed (superseded by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

@@ -5,6 +5,7 @@ declare(strict_types=1);
 namespace LIT\MCP;
 
 use LIT\MCP\Abilities\AbilityRegistrar;
+use LIT\MCP\Abilities\ExposurePolicy;
 use LIT\MCP\ChangeLog\ChangeLog;
 use LIT\MCP\ChangeLog\ChangeLogCleanup;
 use LIT\MCP\ChangeLog\ChangeLogInstaller;
@@ -54,10 +55,15 @@ final class Plugin
             new MediaIntegration($schema, $changeLog, $limits)
         );
 
-        (new AbilityRegistrar($registry))->register();
+        $policy = ExposurePolicy::fromOption();
+        (new AbilityRegistrar($registry, $policy))->register();
         (new ServerRegistrar(
             $registry,
-            new InstructionsBuilder($registry, static fn (): array => $schema->postTypeNames())
+            new InstructionsBuilder($registry, static fn (): array => $schema->postTypeNames(), $policy),
+            null,
+            null,
+            null,
+            $policy
         ))->register();
 
         \WPMedia\MCP\OAuth\Bootstrap::instance();