contextTier=long_context is not applied on startup / no CLI flag for long context tier

## Describe the bug

The CLI accepts `contextTier: "long_context"` in `~/.copilot/settings.json`, but new/default sessions launched non-interactively do not reliably start with the long context tier. The session starts with the smaller/default context even when settings specify `model`, `effortLevel`, and `contextTier`.

Selecting the same model interactively through `/model` switches the session from `gpt-5.5 (xhigh)` to `gpt-5.5 (xhigh) (1.1M context)`, which suggests the context tier is recognized by the model picker but not applied during startup/model resolution.

This blocks autonomous or automatically launched sessions that start with an initial prompt, because those sessions cannot interactively choose the long-context tier after launch.

## Affected version

GitHub Copilot CLI 1.0.52-4

Observed installed package: `1.0.52-4` on Windows.

## Steps to reproduce the behavior

1. Configure `~/.copilot/settings.json` with:

```json
{
"model": "gpt-5.5",
"effortLevel": "xhigh",
"contextTier": "long_context"
}
```

2. Start a new default Copilot CLI session, or launch an automated session with an initial prompt.
3. Run `/context` or inspect the displayed model/context information.
4. Observe that the session uses the smaller/default context rather than the long-context tier.
5. In an interactive session, run `/model` and select the same model/tier. The display changes to the long-context variant, for example `gpt-5.5 (xhigh) (1.1M context)`.

## Expected behavior

A new session should honor `contextTier: "long_context"` from `settings.json` during startup/model resolution. For automation, there should also be a launch flag such as:

```bash
copilot --model gpt-5.5 --reasoning-effort xhigh --context-tier long_context
```

or equivalent.

## Additional context

I inspected the installed 1.0.52-4 bundle locally and found:

- `contextTier` is schema-recognized with values `"default"` and `"long_context"`.
- The interactive model picker has a `context_tier` phase and writes `contextTier` when selecting the long-context tier.
- The documented CLI flags include `--model` and `--reasoning-effort`, but not a context-tier flag.
- The startup/model-resolution path appears to read `model` and `effortLevel`, while the UI path applies context-tier capability overrides later.
- The plain `/model ` command path appears to clear `contextTier`, while the picker path can write it.

Operating system: Windows 11 / Windows Terminal / PowerShell.