feat: Add orchestration server — REST/WebSocket gateway and C++ agent gRPC client

[kox13]

Closes #7

This pull request implements the communication layer between the dashboard
and C++ agents. A Go server acts as a gateway — exposing REST and WebSocket
endpoints to the dashboard, while communicating with agents over gRPC.
The C++ agent gains a gRPC client stub to connect to the server and stream
metrics in response to received messages.

gRPC agent gateway (Go server)

• Added a GET /api/agents REST endpoint that lists all connected agents and their status
• Added a POST /api/agents/{id}/stream REST endpoint that dispatches a streaming task to a specific agent over a gRPC stream.
• Added a /ws?agent_id={id} WebSocket endpoint that forwards incoming metric frames from the agent to the connected dashboard clients. WebSocket connections are per-agent — the dashboard subscribes to a specific agent's stream.
• Implemented a gRPC server that receives metric streams from agents and routes them to the appropriate active WebSocket connection. Metric streaming starts on task dispatch, not on connection.

gRPC client (C++ agent)

• Added a gRPC client implementation in the agent that connects to the server on startup and opens a bidirectional stream.
• On receiving a task from the server, the agent begins collecting and streaming metrics back through the open gRPC stream.
• Proto definitions were extended to cover task dispatch and metric stream message types shared between Go and C++.

These changes introduce a complete request/response + streaming pipeline between the dashboard and agents, laying the foundation for real-time metric observability.