fix: 4 runtime bugs — forge cancel (exec.CommandContext), config key (default_agent_config), sign on Linux (osslsigncode), garble on all platforms

server/config.go

@@ -405,7 +405,8 @@ func mergeConfigExplicit(dst, src *Config, present map[string]json.RawMessage) {
 		}
 	}
 
-	if has("default_agent") {
+	// The JSON struct tag is "default_agent_config" — must match exactly.
+	if has("default_agent_config") {
 		if src.DefaultAgent.Threads != 0 {
 			dst.DefaultAgent.Threads = src.DefaultAgent.Threads
 		}