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/guide/agent-model-matching.md

@@ -190,7 +190,7 @@ See the [Orchestration System Guide](./orchestration.md) for how agents dispatch
   "categories": {
     "quick": { "model": "opencode/gpt-5-nano" },
     "unspecified-low": { "model": "anthropic/claude-sonnet-4-6" },
-    "unspecified-high": { "model": "openai/gpt-5.4", "variant": "high" },
+    "unspecified-high": { "model": "openai/gpt-5.4-high" },
     "visual-engineering": {
       "model": "google/gemini-3.1-pro",
       "variant": "high",