[Bug] Session title auto-generation fails silently with GitHub Copilot provider #24845

Description

Session titles are never auto-generated when using the github-copilot provider. All sessions stay as "New session - " indefinitely.

Root Cause

The title agent uses getSmallModel() which resolves to gpt-5-mini for the GitHub Copilot provider. OpenCode sends reasoning_effort: "minimal" but the GitHub Copilot API only accepts [low, medium, high], causing a 400 error. The error is silently swallowed by Effect.ignore.

Error from logs

ERROR service=llm providerID=github-copilot modelID=claude-opus-4.6 sessionID=ses_... small=false agent=... error={
  "statusCode": 400,
  "responseBody": "{\"error\":{\"message\":\"reasoning_effort \\\"minimal\\\" is not supported by model gpt-5-mini; supported values: [low medium high]\",\"code\":\"invalid_reasoning_effort\"}}\n"
} stream error

ERROR service=session.prompt error=No output generated. Check the stream for errors. failed to generate title

Steps to Reproduce

1. Configure OpenCode with github-copilot provider
2. Start a new session and send a message
3. Session title remains "New session - " and is never auto-renamed

Expected Behavior

Session title should be auto-generated after the first user message.

Environment

• OpenCode version: 1.3.5
• Provider: github-copilot
• Small model selected: gpt-5-mini

Relevant Code

packages/opencode/src/session/prompt.ts - ensureTitle() function (around line 162) calls the title agent with small: true, which picks gpt-5-mini. The reasoning_effort transform sends "minimal" which is incompatible with the GitHub Copilot API.