fix(vscode): use default model from backend instead of hardcoded kilo/auto

[doozie-akshay]

Summary

The extension was hardcoded to use kilo/auto as the default model. Now it uses the default model from the backend response, falling back to kilo/auto only when no default is available.

Problem

• Default model was hardcoded to kilo/auto
• Users couldn't use their configured default model from CLI

Solution

Changed from:

const providerID = config.get<string>("providerID", "kilo")
const modelID = config.get<string>("modelID", "kilo/auto")

To:

const providerID = configProviderID || Object.keys(response.default)[0] || "kilo"
const modelID = configModelID || response.default[providerID] || "kilo/auto"

Now uses:

1. User's config if set
2. Backend's default model for the provider
3. Falls back to kilo/auto only as last resort

Fixes #265