refactor(models): update agent/category fallback chains

- quick: replace openai fallback with opencode/grok-code
- writing: add zai-coding-plan/glm-4.7 between sonnet and gpt
- unspecified-low: gpt-5.2 → gpt-5.2-codex (medium)
- Sisyphus: add zai/glm-4.7 before openai, use gpt-5.2-codex (medium)
- Momus & Metis: add variant 'max' to gemini-3-pro
- explore: simplify to haiku (anthropic/opencode) → grok-code (opencode)

src/cli/model-fallback.ts

@@ -139,12 +139,12 @@ export function generateModelConfig(config: InstallConfig): GeneratedOmoConfig {
       continue
     }
 
-    // Special case: explore has custom Gemini → Claude → Grok logic
+    // Special case: explore uses Claude haiku → OpenCode grok-code
     if (role === "explore") {
-      if (avail.native.gemini) {
-        agents[role] = { model: "google/gemini-3-flash-preview" }
-      } else if (avail.native.claude && avail.isMaxPlan) {
+      if (avail.native.claude) {
         agents[role] = { model: "anthropic/claude-haiku-4-5" }
+      } else if (avail.opencodeZen) {
+        agents[role] = { model: "opencode/claude-haiku-4-5" }
       } else {
         agents[role] = { model: "opencode/grok-code" }
       }