feat(models): add gpt-5.5 fallback specs and default

Add gpt-5.5 to the officialModelSpecs fallback table (mirroring
gpt-5.4's context window/output token values since the Codex CLI
does not yet report specs for 5.5) and promote it to the fallback
defaultModelID used when the model/list response does not flag any
model with isDefault.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Savid

internal/subprocess/appserver_adapter.go

@@ -984,6 +984,10 @@ var officialModelSpecs = map[string]modelSpec{
 		ContextWindow:   1050000,
 		MaxOutputTokens: 128000,
 	},
+	"gpt-5.5": {
+		ContextWindow:   1050000,
+		MaxOutputTokens: 128000,
+	},
 }
 
 var runtimeModelSpecs = map[string]modelSpec{
@@ -1065,7 +1069,7 @@ func (a *AppServerAdapter) listAllModels(ctx context.Context) (*modelListResult,
 
 // defaultModelID is the model marked as default when the API does
 // not flag any model with isDefault.
-const defaultModelID = "gpt-5.3-codex"
+const defaultModelID = "gpt-5.5"
 
 // ensureDefaultModel sets isDefault on the preferred model when no
 // model in the list has isDefault set to true.

internal/subprocess/appserver_adapter_test.go

@@ -3111,12 +3111,12 @@ func TestTranslateNotification_ErrorMethod(t *testing.T) {
 func TestEnsureDefaultModel(t *testing.T) {
 	t.Parallel()
 
-	t.Run("no default set picks gpt-5.3-codex", func(t *testing.T) {
+	t.Run("no default set picks gpt-5.5", func(t *testing.T) {
 		t.Parallel()
 
 		models := []map[string]any{
 			{"id": "gpt-4.1-mini"},
-			{"id": "gpt-5.3-codex"},
+			{"id": "gpt-5.5"},
 			{"id": "o3-pro"},
 		}