feat: inject mode changes into engine conversation history

Engine.InjectMessage() appends messages to history without triggering
a turn. When permission mode or incognito changes, the notification
is injected as a user+assistant pair so the model sees it as context.

Fixes: model now knows permissions changed and will retry tool calls
instead of remembering old denials from previous mode.

internal/engine/engine.go

@@ -107,6 +107,13 @@ func (e *Engine) History() []message.Message {
 	return e.history
 }
 
+// InjectMessage appends a message to conversation history without triggering a turn.
+// Used for system notifications (permission mode changes, incognito toggles) that
+// the model should see as context in subsequent turns.
+func (e *Engine) InjectMessage(msg message.Message) {
+	e.history = append(e.history, msg)
+}
+
 // Usage returns cumulative token usage.
 func (e *Engine) Usage() message.Usage {
 	return e.usage