feat(delegate-task): use explicit high variant for unspecified-high category

- Update DEFAULT_CATEGORIES to use 'openai/gpt-5.4-high' directly instead of separate model + variant
- Add helper functions (isExplicitHighModel, getExplicitHighBaseModel) to preserve explicit high models during fuzzy matching
- Update category resolver to avoid collapsing explicit high models to base model + variant pair
- Update tests to verify explicit high model handling in both background and sync modes
- Update documentation examples to reflect new configuration

🤖 Generated with OhMyOpenCode assistance

docs/reference/configuration.md

@@ -100,7 +100,7 @@ Here's a practical starting configuration:
     "unspecified-low": { "model": "anthropic/claude-sonnet-4-6" },
 
     // unspecified-high — complex work
-    "unspecified-high": { "model": "openai/gpt-5.4", "variant": "high" },
+    "unspecified-high": { "model": "openai/gpt-5.4-high" },
 
     // writing — docs/prose
     "writing": { "model": "google/gemini-3-flash" },