refactor(config): centralize env loading and enforce typed access

- Centralize environment variable loading in config.MustLoad
- Parse and validate all env vars once at initialization
- Make config fields private and read-only
- Remove public Getenv usage in favor of typed accessors
- Improve validation and initialization order
- Normalize enum naming to be idiomatic and avoid constant collisions

session/interaction/model.go

@@ -41,7 +41,7 @@ type model struct {
 }
 
 func (m *model) getTunnelURL() string {
-	if m.tunnelType == types.HTTP {
+	if m.tunnelType == types.TunnelTypeHTTP {
 		return buildURL(m.protocol, m.interaction.slug.String(), m.domain)
 	}
 	return fmt.Sprintf("tcp://%s:%d", m.domain, m.port)