chore: upgrade all GPT model references to gpt-5.4

GPT no longer has separate codex variants (gpt-5.x-codex), unified as
gpt-5.4. Updated ~26 references across 8 files:

- Codex provider presets: default param + 12 vendor configs
- Codex custom template default model
- Universal provider default Codex model (gpt-4o → gpt-5.4)
- OpenCode @ai-sdk/openai: consolidated 7 old models into single gpt-5.4
- OpenCode RightCode: removed gpt-5.2-codex entry
- OMO agent/category recommended models (7 references)
- Stream check default Codex model
- Universal form fallback and placeholder
- Codex form placeholder text

src/components/usage/ModelTestConfigPanel.tsx

@@ -24,7 +24,7 @@ export function ModelTestConfigPanel() {
     maxRetries: "2",
     degradedThresholdMs: "6000",
     claudeModel: "claude-haiku-4-5-20251001",
-    codexModel: "gpt-5.1-codex@low",
+    codexModel: "gpt-5.4@low",
     geminiModel: "gemini-3-pro-preview",
     testPrompt: "Who are you?",
   });