fix: race condition causing prompt.set undefined error

[chriswritescode-dev]

Summary

Fixes a race condition where onMount() was called before the prompt ref was initialized, causing a "undefined is not an object (evaluating 'prompt.set')" error when starting a new session from inside an existing session.

Root Cause

In packages/opencode/src/cli/cmd/tui/routes/home.tsx:79-90, the onMount() hook attempted to call prompt.set() before the ref had been assigned by the <Prompt ref={(r) => { prompt = r }} /> component. This resulted in prompt being undefined during execution.

Fix

• Changed from onMount() to createEffect() which re-evaluates when reactive dependencies change
• Added a guard if (once || !prompt) return to check if the prompt ref exists

Fixes #7870