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

docs/guides/overview.md

@@ -18,7 +18,7 @@ des verbundenen WordPress-Benutzers.
 ## Werkzeuge
 
 Direkt sichtbar: `lit-guide`, `lit-site-status`, `lit-content-types`, `lit-content-query`,
-`lit-content-get`, `lit-content-create`, `lit-content-update`, `lit-post-publish`.
+`lit-content-get`, `lit-content-create`, `lit-content-update`, `lit-post-publish`, `lit-media-search`.
 
 Über `mcp-adapter-discover-abilities` / `mcp-adapter-execute-ability` (Ability-Namen `lit/…`; in Guides und
 Hinweisen immer so geschrieben):