No warning when selected model is incompatible with xhigh/Ultracode effort level

Description

When the model is set to opusplan (Opus in plan mode, Sonnet otherwise), there is no warning that this configuration may be incompatible with xhigh/Ultracode. The incompatibility is only surfaced when the user explicitly runs /effort, with a vague message:

Ultracode needs dynamic workflows enabled (see /config) and an xhigh-capable model.

This message does not identify which model is required, nor whether the currently selected model qualifies.

Steps to Reproduce

1. Run /model opusplan
2. Run /effort xhigh (or attempt to enable Ultracode)
3. Observe the error message — no actionable guidance on which model to select

Expected Behavior

• Warn at config-change time if the selected model cannot satisfy xhigh/Ultracode requirements
• At minimum, the error message should name a concrete compatible model (e.g. "requires claude-opus-4-8 or equivalent")

Actual Behavior

The incompatibility is silently accepted at config time and only surfaces at use time with a non-actionable error message.

Environment

• Platform: Linux (Fedora 44)
• Claude Code version: current