fix: use cache file for model availability instead of SDK calls

- Changed fetchAvailableModels to read from ~/.cache/opencode/models.json
- Prevents plugin startup hanging caused by SDK client.config.providers() call
- Updated doctor model-resolution check to show available models from cache
- Added cache info display: provider count, model count, refresh command

src/cli/doctor/checks/model-resolution.test.ts

@@ -115,8 +115,9 @@ describe("model-resolution check", () => {
       // #then: Details should contain agent/category resolution info
       expect(result.details).toBeDefined()
       expect(result.details!.length).toBeGreaterThan(0)
-      // Should have Current Models header and sections
-      expect(result.details!.some((d) => d.includes("Current Models"))).toBe(true)
+      // Should have Available Models and Configured Models headers
+      expect(result.details!.some((d) => d.includes("Available Models"))).toBe(true)
+      expect(result.details!.some((d) => d.includes("Configured Models"))).toBe(true)
       expect(result.details!.some((d) => d.includes("Agents:"))).toBe(true)
       expect(result.details!.some((d) => d.includes("Categories:"))).toBe(true)
       // Should have legend