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/momus.ts

@@ -17,8 +17,6 @@ import { createAgentToolRestrictions } from "../shared/permission-compat"
  * implementation.
  */
 
-const DEFAULT_MODEL = "openai/gpt-5.2"
-
 export const MOMUS_SYSTEM_PROMPT = `You are a work plan review expert. You review the provided work plan (.sisyphus/plans/{name}.md in the current working project directory) according to **unified, consistent criteria** that ensure clarity, verifiability, and completeness.
 
 **CRITICAL FIRST RULE**:
@@ -391,7 +389,7 @@ Use structured format, **in the same language as the work plan**.
 **FINAL REMINDER**: You are a DOCUMENTATION reviewer, not a DESIGN consultant. The author's implementation direction is SACRED. Your job ends at "Is this well-documented enough to execute?" - NOT "Is this the right approach?"
 `
 
-export function createMomusAgent(model: string = DEFAULT_MODEL): AgentConfig {
+export function createMomusAgent(model: string): AgentConfig {
   const restrictions = createAgentToolRestrictions([
     "write",
     "edit",
@@ -416,7 +414,6 @@ export function createMomusAgent(model: string = DEFAULT_MODEL): AgentConfig {
   return { ...base, thinking: { type: "enabled", budgetTokens: 32000 } } as AgentConfig
 }
 
-export const momusAgent = createMomusAgent()
 
 export const momusPromptMetadata: AgentPromptMetadata = {
   category: "advisor",