feat(deep): upgrade default model from gpt-5.3-codex to gpt-5.4

Deep category now uses gpt-5.4 as its default model across all providers
(openai, github-copilot, venice, opencode), matching Hephaestus's GPT 5.4
upgrade. The requiresModel constraint is removed since gpt-5.4 is widely
available. Adds openai/gpt-5.3-codex -> openai/gpt-5.4 config migration
for existing user configs. Deep category prompt optimized for GPT 5.4's
stronger native capabilities (leaner, less verbose).

src/shared/model-requirements.ts

@@ -222,8 +222,8 @@ export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
   deep: {
     fallbackChain: [
       {
-        providers: ["openai", "opencode"],
-        model: "gpt-5.3-codex",
+        providers: ["openai", "github-copilot", "venice", "opencode"],
+        model: "gpt-5.4",
         variant: "medium",
       },
       {
@@ -237,7 +237,6 @@ export const CATEGORY_MODEL_REQUIREMENTS: Record<string, ModelRequirement> = {
         variant: "high",
       },
     ],
-    requiresModel: "gpt-5.3-codex",
   },
   artistry: {
     fallbackChain: [