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/models_fetch.rs

@@ -1475,7 +1475,7 @@ mod tests {
     /// short-circuits BEFORE any HTTP — there is NO mock `/models` server, yet
     /// the fetch returns exactly the 4 compiled-in models keyed
     /// `openai-codex/<slug>` on the Responses backend, ctx 272000, each exposing
-    /// its exact reasoning efforts (incl. the codex-only `xhigh`/`max`/`ultra`).
+    /// its exact reasoning efforts (incl. the codex-only `xhigh`/`max`).
     /// A BOGUS base URL confirms no live `/models` request is attempted (it would
     /// otherwise fail against an unroutable host).
     #[tokio::test(flavor = "multi_thread")]
@@ -1556,10 +1556,10 @@ mod tests {
                 .iter()
                 .map(|o| o.id.as_str())
                 .collect::<Vec<_>>(),
-            vec!["low", "medium", "high", "xhigh", "max", "ultra"],
-            "sol exposes the full codex effort menu incl. ultra"
+            vec!["low", "medium", "high", "xhigh", "max"],
+            "sol exposes the full codex effort menu"
         );
-        // gpt-5.5 tops out at xhigh (no max/ultra).
+        // gpt-5.5 tops out at xhigh (no max).
         let five_five = result
             .models
             .iter()

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()
 }