feat(Mountain): extend Vine gRPC protocol with full extension host API

Add comprehensive gRPC service definitions and stubs for VS Code extension
host capabilities across CocoonService. This expands the protocol with:

- Language Features: Document/workspace symbols, highlights, rename, formatting
  (document/range/on-type), signature help, code lenses, folding ranges,
  selection ranges, semantic tokens, inlay hints, type/call hierarchy,
  linked editing ranges
- Window Operations: Quick pick, input box, progress reporting, webview
  messaging, external URI handling
- File System: Delete, rename, copy, create directory operations
- Output Channels: Create, append, clear, show, dispose
- Tasks: Register provider, execute, terminate
- Authentication: Session management, provider registration
- Extensions: Get extension info, list all extensions
- Terminal: Resize
- Configuration: Get configuration values

All new methods include stub implementations in CocoonServiceImpl returning
empty/default responses. Full implementation awaits ProviderRegistry in
MountainEnvironment.

Part of the extension host capability expansion for Land's Cocoon sidecar.

Author: NikolaRHristov