fix(cli): add -preview suffix for GitHub Copilot Gemini model names

GitHub Copilot uses gemini-3-pro-preview and gemini-3-flash-preview as
the official model identifiers. The CLI installer was generating config
with incorrect names (gemini-3-pro, gemini-3-flash).

Reported by user: the install command was creating config with wrong
model names that don't work with GitHub Copilot API.

src/cli/model-fallback.ts

@@ -75,6 +75,8 @@ function transformModelForProvider(provider: string, model: string): string {
       .replace("claude-sonnet-4-5", "claude-sonnet-4.5")
       .replace("claude-haiku-4-5", "claude-haiku-4.5")
       .replace("claude-sonnet-4", "claude-sonnet-4")
+      .replace("gemini-3-pro", "gemini-3-pro-preview")
+      .replace("gemini-3-flash", "gemini-3-flash-preview")
   }
   return model
 }