Session title silently empty when the title model is a reasoning model (returns only a `<think>` block)

Description

ensureTitle() generates the session title with the small model (small: true).
When that model is a reasoning model (e.g. GLM-4.6 / Kimi K3 via OpenRouter), the
completion is frequently reasoning-only: after the <think>…</think> strip in
ensureTitle, the remaining text is empty. The function then hits if (!cleaned) return
and the session silently keeps its default New session - <timestamp> title.

It is intermittent (a reasoning model returns usable text on some turns, think-only on
others), which matches the "happens at times" reports below.

Affected code

packages/opencode/src/session/prompt.ts — ensureTitle():

const cleaned = text
  .replace(/<think>[\s\S]*?<\/think>\s*/g, "")
  .split("\n").map(l => l.trim()).find(l => l.length > 0)
if (!cleaned) return   // reasoning-only output -> empty -> no title, silently

Steps to reproduce

1. Set the small/title model to a reasoning model (e.g. openrouter/z-ai/glm-4.6).
2. Start a new session and send a normal message.
3. On turns where the title model returns only reasoning, the title stays
   New session - <timestamp>.

Root cause

ensureTitle withholds the title whenever the model output is empty, instead of
falling back to a deterministic source. Reasoning-only output is the common trigger.

Related open issues

Same symptom: #13710, #31592, #34257, #30662.
Same underlying family (reasoning-only / empty output treated as a successful, empty
result): #37372, #41469, #41571.

Proposed fix

Fall back to the first user message when the model output is empty — the title is then
never withheld, regardless of why the model returned nothing. I have this change ready
and will open a PR referencing this issue.