feat(api): implement base gRPC server over UDS

[rplunger]

Goal

Set up the minimal gRPC server over Unix Domain Sockets that exposes a health/status endpoint. This is the first API adapter proving the gRPC-over-UDS architecture from ADR-004.

Scope

In scope:

• tonic-based gRPC server listening on a UDS path
• Health check / system status RPC (basic GetStatus)
• Feature-gated behind api_grpc_uds
• Proto definition for the health/status service

Out of scope:

• gRPC over TCP (M0.3)
• REST/HTTP endpoints (M0.2)
• MCP server (M0.2)
• Full CRUD API (M0.2)

Done when

• gRPC server starts on UDS and accepts connections
• GetStatus RPC returns engine status
• Proto file committed and builds via tonic-build
• Integration test connects and calls GetStatus
• cargo clippy clean