refactor: simplify nil channel handling in Start()

Remove unnecessary channel creation for nil initialPromptReady.
In Go's select statement, nil channel cases are simply skipped
(never selected), so we don't need to create a new channel that
blocks forever - the nil channel already has the desired behavior.

Addresses PR review feedback.

Author: johnstcn

lib/screentracker/pty_conversation.go

@@ -137,12 +137,8 @@ func (c *PTYConversation) Start(ctx context.Context) {
 		ticker := c.cfg.Clock.NewTicker(c.cfg.SnapshotInterval)
 		defer ticker.Stop()
 
-		// Create a nil channel if no initial prompt - select will never receive from it
+		// If nil, the select case below is simply never chosen (nil channels are skipped in select)
 		initialPromptReady := c.initialPromptReady
-		if initialPromptReady == nil {
-			initialPromptReady = make(chan struct{})
-			// Don't close it - we want it to block forever
-		}
 
 		for {
 			select {