Add $PORT and $PORT_GRPC env var overrides for MCP agent port assignment

After TOML loading and generic env overrides, config.Load now checks
$PORT and $PORT_GRPC and overrides ServerConfig.ListenAddr and
ServerConfig.GRPCAddr respectively. These take precedence over all
other config sources because they represent agent-assigned authoritative
port bindings.

Handles both Base embedding (MCR, MCNS, MCAT) and direct ServerConfig
embedding (Metacrypt) via struct tree walking.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

config/config_test.go

@@ -401,3 +401,109 @@ func TestEmptyEnvPrefix(t *testing.T) {
 		t.Fatalf("ListenAddr = %q, want %q", cfg.Server.ListenAddr, ":8443")
 	}
 }
+
+// directServerConfig embeds ServerConfig without Base (Metacrypt pattern).
+type directServerConfig struct {
+	Server ServerConfig `toml:"server"`
+	Extra  string       `toml:"extra"`
+}
+
+func TestPortEnvOverridesListenAddr(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestPortGRPCEnvOverridesGRPCAddr(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("PORT_GRPC", "9998")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.GRPCAddr != ":9998" {
+		t.Fatalf("GRPCAddr = %q, want %q", cfg.Server.GRPCAddr, ":9998")
+	}
+}
+
+func TestPortEnvOverridesTOMLValue(t *testing.T) {
+	// fullTOML sets listen_addr = ":8443" and grpc_addr = ":9443".
+	path := writeTOML(t, fullTOML)
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT should override TOML)", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestPortEnvOverridesGenericEnv(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+	t.Setenv("TEST_SERVER_LISTEN_ADDR", ":7777")
+	t.Setenv("PORT", "9999")
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":9999" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT should override generic env)", cfg.Server.ListenAddr, ":9999")
+	}
+}
+
+func TestNoPortEnvNoChange(t *testing.T) {
+	path := writeTOML(t, minimalTOML)
+
+	cfg, err := Load[testConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	// minimalTOML sets listen_addr = ":8443", no $PORT set.
+	if cfg.Server.ListenAddr != ":8443" {
+		t.Fatalf("ListenAddr = %q, want %q (TOML value preserved without $PORT)", cfg.Server.ListenAddr, ":8443")
+	}
+}
+
+func TestPortEnvDirectServerConfig(t *testing.T) {
+	// Test the Metacrypt pattern: ServerConfig embedded without Base.
+	toml := `
+[server]
+listen_addr = ":8443"
+tls_cert = "/tmp/cert.pem"
+tls_key = "/tmp/key.pem"
+grpc_addr = ":9443"
+
+extra = "value"
+`
+	path := writeTOML(t, toml)
+	t.Setenv("PORT", "5555")
+	t.Setenv("PORT_GRPC", "5556")
+
+	cfg, err := Load[directServerConfig](path, "TEST")
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+
+	if cfg.Server.ListenAddr != ":5555" {
+		t.Fatalf("ListenAddr = %q, want %q ($PORT on direct ServerConfig)", cfg.Server.ListenAddr, ":5555")
+	}
+	if cfg.Server.GRPCAddr != ":5556" {
+		t.Fatalf("GRPCAddr = %q, want %q ($PORT_GRPC on direct ServerConfig)", cfg.Server.GRPCAddr, ":5556")
+	}
+}