fix(models): drop the ultra reasoning effort, which no backend accepts

The hardcoded Codex catalog advertised `ultra` for gpt-5.6-sol and
gpt-5.6-terra, so selecting it sent `reasoning.effort = "ultra"` and the
Responses endpoint answered 400 with its real menu, which ends at `max`.

The tier was kigi's invention: the enum documented it as codex-only, the
catalog was its only producer, and the upstream model snapshot never
listed it. Removing the variant as well as the catalog entries means
`/effort ultra` is rejected at the TUI instead of parsing and failing on
the wire; a persisted `ultra` degrades to the model default through
`lenient_reasoning_effort_opt`, as designed for vocabulary changes.

crates/codegen/kigi-tui/src/slash/commands/effort_levels.rs

@@ -25,7 +25,6 @@ pub(crate) fn effort_description(level: ReasoningEffort) -> &'static str {
         ReasoningEffort::High => "Heavy reasoning",
         ReasoningEffort::Xhigh => "Extra-heavy reasoning",
         ReasoningEffort::Max => "Maximum reasoning",
-        ReasoningEffort::Ultra => "Ultra reasoning",
     }
 }