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/tools/delegate-task/tools.ts

@@ -11,7 +11,7 @@ import { discoverSkills } from "../../features/opencode-skill-loader"
 import { getTaskToastManager } from "../../features/task-toast-manager"
 import type { ModelFallbackInfo } from "../../features/task-toast-manager/types"
 import { subagentSessions, getSessionAgent } from "../../features/claude-code-session-state"
-import { log, getAgentToolRestrictions } from "../../shared"
+import { log, getAgentToolRestrictions, resolveModel } from "../../shared"
 
 type OpencodeClient = PluginInput["client"]
 
@@ -107,15 +107,15 @@ type ToolContextWithMetadata = {
   metadata?: (input: { title?: string; metadata?: Record<string, unknown> }) => void
 }
 
-function resolveCategoryConfig(
+export function resolveCategoryConfig(
   categoryName: string,
   options: {
     userCategories?: CategoriesConfig
-    parentModelString?: string
-    systemDefaultModel?: string
+    inheritedModel?: string
+    systemDefaultModel: string
   }
-): { config: CategoryConfig; promptAppend: string; model: string | undefined } | null {
-  const { userCategories, parentModelString, systemDefaultModel } = options
+): { config: CategoryConfig; promptAppend: string; model: string } | null {
+  const { userCategories, inheritedModel, systemDefaultModel } = options
   const defaultConfig = DEFAULT_CATEGORIES[categoryName]
   const userConfig = userCategories?.[categoryName]
   const defaultPromptAppend = CATEGORY_PROMPT_APPENDS[categoryName] ?? ""
@@ -124,8 +124,12 @@ function resolveCategoryConfig(
     return null
   }
 
-  // Model priority: user override > category default > parent model (fallback) > system default
-  const model = userConfig?.model ?? defaultConfig?.model ?? parentModelString ?? systemDefaultModel
+  // Model priority: user override > inherited from parent > system default
+  const model = resolveModel({
+    userModel: userConfig?.model,
+    inheritedModel,
+    systemDefault: systemDefaultModel,
+  })
   const config: CategoryConfig = {
     ...defaultConfig,
     ...userConfig,
@@ -421,16 +425,21 @@ ${textContent || "(No text output)"}`
       let categoryModel: { providerID: string; modelID: string; variant?: string } | undefined
       let categoryPromptAppend: string | undefined
 
-      const parentModelString = parentModel
+      const inheritedModel = parentModel
         ? `${parentModel.providerID}/${parentModel.modelID}`
         : undefined
 
       let modelInfo: ModelFallbackInfo | undefined
 
       if (args.category) {
+        // Guard: require system default model for category delegation
+        if (!systemDefaultModel) {
+          return `No default model configured. Set a model in your OpenCode config (model field).`
+        }
+
         const resolved = resolveCategoryConfig(args.category, {
           userCategories,
-          parentModelString,
+          inheritedModel,
           systemDefaultModel,
         })
         if (!resolved) {
@@ -440,11 +449,6 @@ ${textContent || "(No text output)"}`
         // Determine model source by comparing against the actual resolved model
         const actualModel = resolved.model
         const userDefinedModel = userCategories?.[args.category]?.model
-        const categoryDefaultModel = DEFAULT_CATEGORIES[args.category]?.model
-
-        if (!actualModel) {
-          return `No model configured. Set a model in your OpenCode config, plugin config, or use a category with a default model.`
-        }
 
         if (!parseModelString(actualModel)) {
           return `Invalid model format "${actualModel}". Expected "provider/model" format (e.g., "anthropic/claude-sonnet-4-5").`
@@ -454,12 +458,9 @@ ${textContent || "(No text output)"}`
           case userDefinedModel:
             modelInfo = { model: actualModel, type: "user-defined" }
             break
-          case parentModelString:
+          case inheritedModel:
             modelInfo = { model: actualModel, type: "inherited" }
             break
-          case categoryDefaultModel:
-            modelInfo = { model: actualModel, type: "category-default" }
-            break
           case systemDefaultModel:
             modelInfo = { model: actualModel, type: "system-default" }
             break