fix: remove hardcoded model defaults from categories and agents

BREAKING CHANGE: Model resolution overhauled

- Created centralized model-resolver.ts with priority chain:
  userModel → inheritedModel → systemDefaultModel
- Removed model field from all 7 DEFAULT_CATEGORIES entries
- Removed DEFAULT_MODEL constants from 10 agents
- Removed singleton agent exports (use factories instead)
- Made CategoryConfigSchema.model optional
- CLI no longer generates model overrides
- Empty strings treated as unset (uses fallback)

Users must now:
1. Use factory functions (createOracleAgent, etc.) instead of singletons
2. Provide model explicitly or use systemDefaultModel
3. Configure category models explicitly if needed

Fixes model fallback bug where hardcoded defaults overrode
user's OpenCode configured model.

src/shared/migration.ts

@@ -44,7 +44,19 @@ export const HOOK_NAME_MAP: Record<string, string> = {
   "anthropic-auto-compact": "anthropic-context-window-limit-recovery",
 }
 
-// Model to category mapping for auto-migration
+/**
+ * @deprecated LEGACY MIGRATION ONLY
+ * 
+ * This map exists solely for migrating old configs that used hardcoded model strings.
+ * It maps legacy model strings to semantic category names, allowing users to migrate
+ * from explicit model configs to category-based configs.
+ * 
+ * DO NOT add new entries here. New agents should use:
+ * - Category-based config (preferred): { category: "most-capable" }
+ * - Or inherit from OpenCode's config.model
+ * 
+ * This map will be removed in a future major version once migration period ends.
+ */
 export const MODEL_TO_CATEGORY_MAP: Record<string, string> = {
   "google/gemini-3-pro-preview": "visual-engineering",
   "openai/gpt-5.2": "ultrabrain",