feat: add client capability schema

Adds JSON Schema 2020-12 for MCP client capabilities at docs/schemas/client-registry.json

Features:
- Granular capability status (full/partial/unsupported/untested)
- Transport, platform, and authentication tracking
- Metadata fields (vendor, license, conformanceScore, lastVerified)
- Tasks capability with list, cancel, and requests structure (per MCP 2025-11-25 spec)
- Backward compatible with presence-based format

Closes #718

🦉 Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: olaservo