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/shared/migration.test.ts

@@ -370,9 +370,9 @@ describe("shouldDeleteAgentConfig", () => {
 
   test("returns true when all fields match category defaults", () => {
     // #given: Config with fields matching category defaults
+    // Note: DEFAULT_CATEGORIES only has temperature, not model
     const config = {
       category: "visual-engineering",
-      model: "google/gemini-3-pro-preview",
       temperature: 0.7,
     }