feat(grpc): add embedded MCP server lifecycle RPCs

Summary

The daemon's REST API exposes three endpoints for controlling the embedded MCP server:

• GET /api/mcp-server/status — check if the embedded MCP server is running
• POST /api/mcp-server/start — start the embedded MCP server
• POST /api/mcp-server/stop — stop the embedded MCP server

These have no gRPC equivalents. The VS Code extension (and other gRPC clients) cannot manage the embedded MCP server lifecycle.

Proposal

Add three RPCs to the Abbenay service in service.proto:

rpc GetMcpServerStatus(GetMcpServerStatusRequest) returns (GetMcpServerStatusResponse);
rpc StartMcpServer(StartMcpServerRequest) returns (Empty);
rpc StopMcpServer(StopMcpServerRequest) returns (Empty);

Note: these are distinct from StartWebServer/StopWebServer (which control the HTTP dashboard) and from RegisterMcpServer/UnregisterMcpServer (which manage dynamic MCP server connections).

Context

Identified during gRPC config parity audit in #32.

Start with service.proto and compare the existing StartWebServer/StopWebServer and RegisterMcpServer/UnregisterMcpServer RPCs. Trace the REST lifecycle endpoints for the embedded MCP server, then follow the corresponding gRPC service implementation and generated bindings. Done means gRPC clients can query status and start or stop the embedded MCP server.