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

src/tools/delegate-task/constants.ts

@@ -289,7 +289,7 @@ export const DEFAULT_CATEGORIES: Record<string, CategoryConfig> = {
   artistry: { model: "google/gemini-3.1-pro", variant: "high" },
   quick: { model: "anthropic/claude-haiku-4-5" },
   "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" },
   writing: { model: "kimi-for-coding/k2p5" },
 }