feat(migration): auto-upgrade openai/gpt-5.4 to openai/gpt-5.5

Now that oracle, hephaestus, and the deep category ship gpt-5.5-native
prompts and default to gpt-5.5 in their fallback chains, user configs
that pin openai/gpt-5.4 should follow the same upgrade on next load.

The existing migration pipeline (migrateModelVersions) picks this up
automatically from MODEL_VERSION_MAP, records the upgrade in the
sidecar file to stay idempotent, and leaves gpt-5.4-mini / -mini-fast
/ -nano alone since those cost-tier variants are intentional picks
and do not have direct gpt-5.5 counterparts yet.

src/shared/migration/model-versions.ts

@@ -10,6 +10,7 @@ export const MODEL_VERSION_MAP: Record<string, string> = {
   "anthropic/claude-opus-4-6": "anthropic/claude-opus-4-7",
   "anthropic/claude-sonnet-4-5": "anthropic/claude-sonnet-4-6",
   "openai/gpt-5.3-codex": "openai/gpt-5.4",
+  "openai/gpt-5.4": "openai/gpt-5.5",
 }
 
 function migrationKey(oldModel: string, newModel: string): string {