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

tests/Unit/Integrations/Media/MediaIntegrationTest.php

@@ -16,6 +16,7 @@ final class MediaIntegrationTest extends TestCase
 {
     /** Grows with Tasks 2–5 – keep in the order of abilities(). */
     private const EXPECTED = [
+        'lit/media-search',
         'lit/media-get',
     ];