Switch gRPC admin API to Unix socket only, add client package

- Remove TCP listener support from gRPC server; Unix socket is now the
  only transport for the admin API (access controlled via filesystem
  permissions)
- Add standard gRPC health check service (grpc.health.v1.Health)
- Implement MCPROXY_* environment variable overrides for config
- Create client/mcproxy package with full API coverage and tests
- Update ARCHITECTURE.md and dev config (srv/mc-proxy.toml)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/mcproxy/doc.go

@@ -0,0 +1,41 @@
+// Package mcproxy provides a Go client for the mc-proxy gRPC admin API.
+//
+// The client connects to mc-proxy via Unix socket and provides methods
+// for managing routes, firewall rules, and querying server status.
+//
+// # Basic Usage
+//
+//	client, err := mcproxy.Dial("/var/run/mc-proxy.sock")
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//	defer client.Close()
+//
+//	// Get server status
+//	status, err := client.GetStatus(ctx)
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//	fmt.Printf("mc-proxy %s, %d connections\n", status.Version, status.TotalConnections)
+//
+//	// List routes for a listener
+//	routes, err := client.ListRoutes(ctx, ":443")
+//	if err != nil {
+//	    log.Fatal(err)
+//	}
+//	for _, r := range routes {
+//	    fmt.Printf("  %s -> %s\n", r.Hostname, r.Backend)
+//	}
+//
+//	// Add a route
+//	err = client.AddRoute(ctx, ":443", "example.com", "127.0.0.1:8443")
+//
+//	// Add a firewall rule
+//	err = client.AddFirewallRule(ctx, mcproxy.FirewallRuleCIDR, "10.0.0.0/8")
+//
+//	// Check health
+//	health, err := client.CheckHealth(ctx)
+//	if health == mcproxy.HealthServing {
+//	    fmt.Println("Server is healthy")
+//	}
+package mcproxy