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-shell/src/agent/session_config.rs

@@ -60,7 +60,6 @@ fn effort_label(effort: ReasoningEffort) -> String {
         ReasoningEffort::High => "High",
         ReasoningEffort::Xhigh => "X-High",
         ReasoningEffort::Max => "Max",
-        ReasoningEffort::Ultra => "Ultra",
     }
     .to_string()
 }