Create Codex threads on first message after clear

PLAN.md

@@ -249,6 +249,10 @@ That notification retries the waiting queue, so delivery does not depend on a
 later message happening to retry it. A message accepted while initialization or
 recovery is still finding a thread is recorded as queued; the phone can therefore
 reopen without losing the only visible copy before Codex acknowledges it.
+Clearing does not eagerly create that replacement: it forgets the old id and
+the next message starts a thread and its first turn together. An empty replacement
+would become exactly the unresumable id above if the app-server restarted between
+the clear and the next message.
 
 Codex subscription limits come from the CLI's `account/rateLimits/read`
 app-server request on the machine that runs Codex. This keeps login and