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_

Start by reproducing the issue with --context=long_context and /config contextTier long_context, then use /context to compare the selected model context. Trace the CLI's context-tier configuration and propagation to the main session and dispatched sub agents; done means both launch-time and in-session settings select the requested long-context tier.

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