refactor(errs): migrate from gperr.Error to standard Go error interface

This is a large-scale refactoring across the codebase that replaces the custom
`gperr.Error` type with Go's standard `error` interface. The changes include:

- Replacing `gperr.Error` return types with `error` in function signatures
- Using `errors.New()` and `fmt.Errorf()` instead of `gperr.New()` and `gperr.Errorf()`
- Using `%w` format verb for error wrapping instead of `.With()` method
- Replacing `gperr.Subject()` calls with `gperr.PrependSubject()`
- Converting error logging from `gperr.Log*()` functions to zerolog's `.Err().Msg()` pattern
- Update NewLogger to handle multiline error message
- Updating `goutils` submodule to latest commit

This refactoring aligns with Go idioms and removes the dependency on
custom error handling abstractions in favor of standard library patterns.

agent/pkg/agent/config.go

@@ -216,7 +216,7 @@ func (cfg *AgentConfig) InitWithCerts(ctx context.Context, ca, crt, key []byte)
 	cfg.l = log.With().Str("agent", cfg.Name).Logger()
 
 	if err := streamUnsupportedErrs.Error(); err != nil {
-		gperr.LogWarn("agent has limited/no stream tunneling support, TCP and UDP routes via agent will not work", err, &cfg.l)
+		cfg.l.Warn().Err(err).Msg("agent has limited/no stream tunneling support, TCP and UDP routes via agent will not work")
 	}
 
 	if serverVersion.IsNewerThanMajor(cfg.Version) {