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/agents/utils.ts

@@ -9,7 +9,7 @@ import { createFrontendUiUxEngineerAgent, FRONTEND_PROMPT_METADATA } from "./fro
 import { createDocumentWriterAgent, DOCUMENT_WRITER_PROMPT_METADATA } from "./document-writer"
 import { createMultimodalLookerAgent, MULTIMODAL_LOOKER_PROMPT_METADATA } from "./multimodal-looker"
 import { createMetisAgent } from "./metis"
-import { createOrchestratorSisyphusAgent, orchestratorSisyphusAgent } from "./orchestrator-sisyphus"
+import { createOrchestratorSisyphusAgent } from "./orchestrator-sisyphus"
 import { createMomusAgent } from "./momus"
 import type { AvailableAgent } from "./sisyphus-prompt-builder"
 import { deepMerge } from "../shared"
@@ -28,7 +28,9 @@ const agentSources: Record<BuiltinAgentName, AgentSource> = {
   "multimodal-looker": createMultimodalLookerAgent,
   "Metis (Plan Consultant)": createMetisAgent,
   "Momus (Plan Reviewer)": createMomusAgent,
-  "orchestrator-sisyphus": orchestratorSisyphusAgent,
+  // Note: orchestrator-sisyphus is handled specially in createBuiltinAgents()
+  // because it needs OrchestratorContext, not just a model string
+  "orchestrator-sisyphus": createOrchestratorSisyphusAgent as unknown as AgentFactory,
 }
 
 /**
@@ -50,7 +52,7 @@ function isFactory(source: AgentSource): source is AgentFactory {
 
 export function buildAgent(
   source: AgentSource,
-  model?: string,
+  model: string,
   categories?: CategoriesConfig,
   gitMasterConfig?: GitMasterConfig
 ): AgentConfig {
@@ -134,6 +136,10 @@ export function createBuiltinAgents(
   categories?: CategoriesConfig,
   gitMasterConfig?: GitMasterConfig
 ): Record<string, AgentConfig> {
+  if (!systemDefaultModel) {
+    throw new Error("createBuiltinAgents requires systemDefaultModel")
+  }
+
   const result: Record<string, AgentConfig> = {}
   const availableAgents: AvailableAgent[] = []
 
@@ -149,7 +155,7 @@ export function createBuiltinAgents(
     if (disabledAgents.includes(agentName)) continue
 
     const override = agentOverrides[agentName]
-    const model = override?.model
+    const model = override?.model ?? systemDefaultModel
 
     let config = buildAgent(source, model, mergedCategories, gitMasterConfig)