Feature Request: Support OpenAI Flex Tier in Copilot CLI

Describe the feature or problem you'd like to solve

Add support for selecting OpenAI's Flex tier (service_tier: flex) in the Copilot CLI model selector and configuration, allowing users to cut token usage costs by 50% for non-time-critical tasks.

Proposed solution

Motivation

OpenAI Flex tier costs 50% less than Standard tier and according to monitoring data from OpenRouter has roughly 30% higher E2E latency (e.g. 9.02s vs 11.56s for gpt-5.6-sol):

For many agentic tasks small increases of latency do not significantly impact user experience.

Additionally, E2E latency for many agentic tasks depends more on tool call durations than on LLM response times (e.g. large C++ or Gradle builds, running extensive test suits, etc). Using Flex tier for these kinds of tasks would reduce cost without significantly impacting task completion time.

Therefore, offering Flex tier would allow some developers to perform roughly twice as many agentic tasks withing a given AI budget.

Proposed Solution

1. Interactive Model Picker: When selecting an OpenAI model, present a tier selection prompt (Standard vs. Flex) alongside the reasoning effort selection.
2. CLI Flags: Add --tier <standard|flex> (or --service-tier <standard|flex>) for scripted runs and aliases.

Example prompts or workflows

No response

Additional context

No response

ファイル、テスト、エントリーポイントは指定されていません。まず Copilot CLI のモデルピッカー、設定処理、OpenAI リクエストの構築箇所を特定します。モデルと推論負荷の選択がリクエストにどのように渡されるかを追跡し、インタラクティブな選択とスクリプト用の tier フラグの両方で、要求された Standard または Flex の service tier が生成されることを確認します。