What does this PR do?

1. Fixed a bug where lastUser is incorrectly identified in multi-turn situations and exits the loop prematurely without generating a response.
   In the below:
   

   opencode/packages/opencode/src/session/prompt.ts

   Lines 282 to 284 in f834915

   	for (let i = msgs.length - 1; i >= 0; i--) {
   	const msg = msgs[i]
   	if (!lastUser && msg.info.role === "user") lastUser = msg.info as MessageV2.User

   msgs are traversed in reverse order, but msgs is not guaranteed to be chronologically ordered. The loop then identifies the wrong lastUser and the exit condition is incorrectly evaluated.
   Fix: Sort msgs by created timestamp to ensure chronological order

2. Changed the loop exit condition from lexicographical ID comparison to a parent-child relationship check to prevent infinite loops.
   In the below:
   

   opencode/packages/opencode/src/session/prompt.ts

   Lines 296 to 300 in f834915

   	if (
   	lastAssistant?.finish &&
   	!["tool-calls", "unknown"].includes(lastAssistant.finish) &&
   	lastUser.id < lastAssistant.id
   	) {

   even if lastUser and lastAssistant are correctly found, if lastUser.id is lexicographically higher than lastAssistant.id, it does not break the loop and continues the steps infinitely.
   Fix: Change exit condition from ID comparison to lastFinished.parentID === lastUser.id, which properly checks if the finished message is a response to the user message

How did you verify your code works?

It was hard to reproduce the issue, since sometimes the conditions are evaluated as expected by chance. I tested a multi-turn conversation scenario several times by logging msgs and the IDs of messages.