feat(grpc): add embedded MCP server lifecycle RPCs

[cidrblock]

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.