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.go

@@ -144,6 +144,8 @@ func Load[T any](path string, envPrefix string) (*T, error) {
 		applyEnvToStruct(reflect.ValueOf(&cfg).Elem(), envPrefix)
 	}
 
+	applyPortEnv(&cfg)
+
 	applyBaseDefaults(&cfg)
 
 	if err := validateBase(&cfg); err != nil {
@@ -239,6 +241,70 @@ func findBase(cfg any) *Base {
 	return nil
 }
 
+// applyPortEnv overrides ServerConfig.ListenAddr and ServerConfig.GRPCAddr
+// from $PORT and $PORT_GRPC respectively. These environment variables are
+// set by the MCP agent to assign authoritative port bindings, so they take
+// precedence over both TOML values and generic env overrides.
+func applyPortEnv(cfg any) {
+	sc := findServerConfig(cfg)
+	if sc == nil {
+		return
+	}
+
+	if port, ok := os.LookupEnv("PORT"); ok {
+		sc.ListenAddr = ":" + port
+	}
+	if port, ok := os.LookupEnv("PORT_GRPC"); ok {
+		sc.GRPCAddr = ":" + port
+	}
+}
+
+// findServerConfig returns a pointer to the ServerConfig in the config
+// struct. It first checks for an embedded Base (which contains Server),
+// then walks the struct tree via reflection to find any ServerConfig field
+// directly (e.g., the Metacrypt pattern where ServerConfig is embedded
+// without Base).
+func findServerConfig(cfg any) *ServerConfig {
+	if base := findBase(cfg); base != nil {
+		return &base.Server
+	}
+
+	return findServerConfigReflect(reflect.ValueOf(cfg))
+}
+
+// findServerConfigReflect walks the struct tree to find a ServerConfig field.
+func findServerConfigReflect(v reflect.Value) *ServerConfig {
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	if v.Kind() != reflect.Struct {
+		return nil
+	}
+
+	scType := reflect.TypeOf(ServerConfig{})
+	t := v.Type()
+	for i := range t.NumField() {
+		field := t.Field(i)
+		fv := v.Field(i)
+
+		if field.Type == scType {
+			sc, ok := fv.Addr().Interface().(*ServerConfig)
+			if ok {
+				return sc
+			}
+		}
+
+		// Recurse into embedded or nested structs.
+		if fv.Kind() == reflect.Struct && field.Type != scType {
+			if sc := findServerConfigReflect(fv); sc != nil {
+				return sc
+			}
+		}
+	}
+
+	return nil
+}
+
 // applyEnvToStruct recursively walks a struct and overrides field values
 // from environment variables. The env variable name is built from the
 // prefix and the toml tag: PREFIX_SECTION_FIELD (uppercased).