Add --input-has-vars flag to control variable substitution in input

- Add InputHasVars field to ChatRequest struct
- Only process template variables in user input when flag is set
- Fixes issue with Ansible/Jekyll templates that use {{var}} syntax

This change makes template variable substitution in user input opt-in
via the --input-has-vars flag, preserving literal curly braces by
default.

common/domain.go

@@ -12,6 +12,7 @@ type ChatRequest struct {
 	Message          *goopenai.ChatCompletionMessage
 	Language         string
 	Meta             string
+	InputHasVars     bool
 }
 
 type ChatOptions struct {