feat: media-search with missing_alt and unused filters

Ruling Q8 caps the unused scan at 20 candidates per call (not the
brief's 500/50), and each result (search and unused alike) is
filtered to attachments the current user may edit; overview.md lists
media-search as a direct tool (ruling Q2).

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

src/Integrations/Media/MediaIntegration.php

@@ -35,6 +35,7 @@ final class MediaIntegration implements Integration
         $finder = new MediaUsageFinder(fn (): array => $this->schema->mediaFieldKeys());
 
         return [
+            new MediaSearchAbility($presenter, $finder),
             new MediaGetAbility($presenter, $finder),
         ];
     }