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/multimodal-looker.ts

@@ -2,8 +2,6 @@ import type { AgentConfig } from "@opencode-ai/sdk"
 import type { AgentPromptMetadata } from "./types"
 import { createAgentToolAllowlist } from "../shared/permission-compat"
 
-const DEFAULT_MODEL = "google/gemini-3-flash"
-
 export const MULTIMODAL_LOOKER_PROMPT_METADATA: AgentPromptMetadata = {
   category: "utility",
   cost: "CHEAP",
@@ -11,9 +9,7 @@ export const MULTIMODAL_LOOKER_PROMPT_METADATA: AgentPromptMetadata = {
   triggers: [],
 }
 
-export function createMultimodalLookerAgent(
-  model: string = DEFAULT_MODEL
-): AgentConfig {
+export function createMultimodalLookerAgent(model: string): AgentConfig {
   const restrictions = createAgentToolAllowlist(["read"])
 
   return {
@@ -58,4 +54,3 @@ Your output goes straight to the main agent for continued work.`,
   }
 }
 
-export const multimodalLookerAgent = createMultimodalLookerAgent()