feat(model): add GPT-5.5 support for Codex provider (#880)

- Bump Codex provider defaults from gpt-5.4 to gpt-5.5 across all ModelConfigs
- Update codexplan alias to resolve to gpt-5.5
- Add gpt-5.5 and gpt-5.5-mini to model picker with reasoning effort mappings
- Add context window and max output token specs for gpt-5.5 family
- Add gpt-5.5 entries to COPILOT_MODELS registry
- Keep official OpenAI API preset at gpt-5.4 (API availability pending)
- Update codexShim tests to expect gpt-5.5 from codexplan alias

Co-authored-by: OpenClaude <openclaude@gitlawb.com>

src/utils/model/modelOptions.ts

@@ -297,9 +297,9 @@ function getOpusPlanOption(): ModelOption {
 
 function getCodexPlanOption(): ModelOption {
   return {
-    value: 'gpt-5.4',
-    label: 'gpt-5.4',
-    description: 'GPT-5.4 on the Codex backend with high reasoning',
+    value: 'gpt-5.5',
+    label: 'gpt-5.5',
+    description: 'GPT-5.5 on the Codex backend with high reasoning',
   }
 }
 
@@ -313,6 +313,11 @@ function getCodexSparkOption(): ModelOption {
 
 function getCodexModelOptions(): ModelOption[] {
   return [
+    {
+      value: 'gpt-5.5',
+      label: 'gpt-5.5',
+      description: 'GPT-5.5 with high reasoning',
+    },
     {
       value: 'gpt-5.4',
       label: 'gpt-5.4',
@@ -348,6 +353,11 @@ function getCodexModelOptions(): ModelOption[] {
       label: 'gpt-5.1-codex-mini',
       description: 'GPT-5.1 Codex Mini - faster, cheaper',
     },
+    {
+      value: 'gpt-5.5-mini',
+      label: 'gpt-5.5-mini',
+      description: 'GPT-5.5 Mini - faster, cheaper',
+    },
     {
       value: 'gpt-5.4-mini',
       label: 'gpt-5.4-mini',
@@ -679,7 +689,7 @@ export function getModelOptions(fastMode = false): ModelOption[] {
     return filterModelOptionsByAllowlist(options)
   } else if (customModel === 'opusplan') {
     return filterModelOptionsByAllowlist([...options, getOpusPlanOption()])
-  } else if (customModel === 'gpt-5.4') {
+  } else if (customModel === 'gpt-5.5') {
     return filterModelOptionsByAllowlist([...options, getCodexPlanOption()])
   } else if (customModel === 'gpt-5.3-codex-spark') {
     return filterModelOptionsByAllowlist([...options, getCodexSparkOption()])