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/orchestrator-sisyphus.ts

@@ -1458,9 +1458,10 @@ function buildDynamicOrchestratorPrompt(ctx?: OrchestratorContext): string {
     .replace("{SKILLS_SECTION}", skillsSection)
 }
 
-const DEFAULT_MODEL = "anthropic/claude-sonnet-4-5"
-
-export function createOrchestratorSisyphusAgent(ctx?: OrchestratorContext): AgentConfig {
+export function createOrchestratorSisyphusAgent(ctx: OrchestratorContext): AgentConfig {
+  if (!ctx.model) {
+    throw new Error("createOrchestratorSisyphusAgent requires a model in context")
+  }
   const restrictions = createAgentToolRestrictions([
     "task",
     "call_omo_agent",
@@ -1469,7 +1470,7 @@ export function createOrchestratorSisyphusAgent(ctx?: OrchestratorContext): Agen
     description:
       "Orchestrates work via delegate_task() to complete ALL tasks in a todo list until fully done",
     mode: "primary" as const,
-    model: ctx?.model ?? DEFAULT_MODEL,
+    model: ctx.model,
     temperature: 0.1,
     prompt: buildDynamicOrchestratorPrompt(ctx),
     thinking: { type: "enabled", budgetTokens: 32000 },
@@ -1478,8 +1479,6 @@ export function createOrchestratorSisyphusAgent(ctx?: OrchestratorContext): Agen
   } as AgentConfig
 }
 
-export const orchestratorSisyphusAgent: AgentConfig = createOrchestratorSisyphusAgent()
-
 export const orchestratorSisyphusPromptMetadata: AgentPromptMetadata = {
   category: "advisor",
   cost: "EXPENSIVE",