EXT-2: LocalExtensionRegistry

[ZhuchkaTriplesix]

Epic: #206
Depends on: EXT-1

Goal

Implement a registry that scans ~/.querya/extensions/ for installed extensions.

Implementation

• Scan subdirectories of the extensions folder for manifest.json files.
• Parse discovered manifests into ExtensionManifest objects.
• Handle malformed manifests gracefully (log error, skip extension).
• Provide a stream or list of available extensions to the app.