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>