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/migration/model-versions.ts

@@ -8,6 +8,7 @@
 export const MODEL_VERSION_MAP: Record<string, string> = {
   "anthropic/claude-opus-4-5": "anthropic/claude-opus-4-6",
   "anthropic/claude-sonnet-4-5": "anthropic/claude-sonnet-4-6",
+  "openai/gpt-5.3-codex": "openai/gpt-5.4",
 }
 
 function migrationKey(oldModel: string, newModel: string): string {