/model silently hides Extra High for GPT-5.4 without explaining tier gating in Copilot CLI 1.0.36

### Describe the bug

Issue #2725 was closed with the explanation that `Extra High` / `xhigh` reasoning effort is intentionally restricted to Business and Enterprise plan tiers.

However, in the current CLI this still presents as a confusing and inconsistent experience:

- `/model` -> `GPT-5.4` only shows `Low / Medium / High`
- the picker gives no explanation that `Extra High` is being hidden because of plan-tier gating
- current client bundles still appear to contain `xhigh`-related labels/descriptions/handling paths, which makes this look like a silent UI filter rather than a clearly communicated product restriction

So the problem here is not "please add xhigh back", but rather:

- the restriction is silently applied in the picker
- the CLI does not clearly explain why the option is missing
- this is easy to interpret as a broken or inconsistent model selector

Related issues:
- #2725
- #2739

### Affected version

GitHub Copilot CLI 1.0.36

### Steps to reproduce the behavior

1. Start `copilot`
2. Run `/model`
3. Select `GPT-5.4`
4. Open the reasoning effort picker
5. Observe that only `Low / Medium / High` are shown
6. Observe that there is no inline explanation that `Extra High` is hidden due to plan-tier restrictions

### Expected behavior

If `Extra High` is intentionally unavailable for the current account tier, the CLI should make that explicit.

For example, any of these would be much clearer than silently omitting it:

- show `Extra High` as disabled with a short explanation
- show a short message in the picker that it is restricted by plan tier
- document the restriction in a place the CLI can point to
- ensure the UI/messaging is consistent with the actual supported-effort metadata exposed by the client

### Additional context

Environment:
- Windows
- GitHub Copilot CLI 1.0.36

I am **not** reporting this as "xhigh definitely works and should be re-enabled".
This report is specifically about the current CLI UX being ambiguous and easy to misinterpret.

After reading #2725, I understand the intended restriction.
The problem is that the current `/model` picker still hides the option without clearly telling the user that this is a plan-tier gate rather than a broken selector.