config option contextTier does nothing

### Describe the bug

Context Tier config option does not seem to actually impact the agent that the cli is launched with, nor any despatched sub agents; until you have manually used the model picker to select a model with long context, THEN the main session and sub agents correctly use long context

### Affected version

GitHub Copilot CLI 1.0.61.

### Steps to reproduce the behavior

Ensure previous copilot cli session was running with a model with default context window selected

Launch the cli with --context=long_context
OR
launch a session with no options and then:
/config contextTier long_context

run `/context` to see usage, the model context shows the default value (not the extended option)

### Expected behavior

launching cli with context option or setting the context tier via config changes the context tier for the model in use and crucially any despatched sub agents

### Additional context

_No response_

先使用 --context=long_context 和 /config contextTier long_context 重現問題，然後使用 /context 比較所選模型的 context。追蹤 CLI 的 context tier 設定及其傳遞至主要工作階段和已派送子代理的流程；當啟動時和工作階段內的設定都選取要求的 long-context tier 時，即表示完成。

由索引模型根據 Issue 內容生成。