fix: update plugin.json link to new location in .claude-plugin/

Author: davideferigato

README.md

@@ -272,7 +272,7 @@ To package MigraAPI as a Claude Code plugin:
    /plugin install migrapi@your-org/skills
    ```
 
-See [`plugin.json`](plugin.json) for metadata.
+See [`plugin.json`](.claude-plugin/plugin.json) for metadata.
 
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