docs: add V2 SDK impact analysis for migration planning

Analysis of how MCP TypeScript SDK V2 changes affect ext-apps:
- Protocol class signature and abstract→concrete changes
- Handler registration API changes (schema→string)
- Schema objects no longer in public API
- Import path migration from monolith to monorepo packages
- Complete inventory of ext-apps SDK dependencies
- Proposed migration strategy (vendor short-term, ProtocolSpec long-term)

Author: claude