fix(provider-cache): extract models from provider.list().all response

OpenCode SDK does not expose client.model.list API. This caused the
provider-models cache to always be empty (models: {}), which in turn
caused delegate-task categories with requiresModel (e.g., 'deep',
'artistry') to fail with misleading 'Unknown category' errors.

Changes:
- connected-providers-cache.ts: Extract models from provider.list()
  response's .all array instead of calling non-existent client.model.list
- category-resolver.ts: Distinguish between 'unknown category' and
  'model not available' errors with clearer error messages
- Add comprehensive tests for both fixes

Bug chain:
client.model?.list is undefined -> empty cache -> isModelAvailable
returns false for requiresModel categories -> null returned from
resolveCategoryConfig -> 'Unknown category' error (wrong message)

src/tools/delegate-task/category-resolver.ts

@@ -29,7 +29,10 @@ export async function resolveCategoryExecution(
 
   const availableModels = await getAvailableModelsForDelegateTask(client)
 
-  const resolved = resolveCategoryConfig(args.category!, {
+  const categoryName = args.category!
+  const categoryExists = DEFAULT_CATEGORIES[categoryName] !== undefined || userCategories?.[categoryName] !== undefined
+
+  const resolved = resolveCategoryConfig(categoryName, {
     userCategories,
     inheritedModel,
     systemDefaultModel,
@@ -37,6 +40,27 @@ export async function resolveCategoryExecution(
   })
 
   if (!resolved) {
+    const requirement = CATEGORY_MODEL_REQUIREMENTS[categoryName]
+    const allCategoryNames = Object.keys({ ...DEFAULT_CATEGORIES, ...userCategories }).join(", ")
+
+    if (categoryExists && requirement?.requiresModel) {
+      return {
+        agentToUse: "",
+        categoryModel: undefined,
+        categoryPromptAppend: undefined,
+        modelInfo: undefined,
+        actualModel: undefined,
+        isUnstableAgent: false,
+        error: `Category "${categoryName}" requires model "${requirement.requiresModel}" which is not available.
+
+To use this category:
+1. Connect a provider with this model: ${requirement.requiresModel}
+2. Or configure an alternative model in your oh-my-opencode.json for this category
+
+Available categories: ${allCategoryNames}`,
+      }
+    }
+
     return {
       agentToUse: "",
       categoryModel: undefined,
@@ -44,7 +68,7 @@ export async function resolveCategoryExecution(
       modelInfo: undefined,
       actualModel: undefined,
       isUnstableAgent: false,
-      error: `Unknown category: "${args.category}". Available: ${Object.keys({ ...DEFAULT_CATEGORIES, ...userCategories }).join(", ")}`,
+      error: `Unknown category: "${categoryName}". Available: ${allCategoryNames}`,
     }
   }