fix: skip API key validation for free opencode models

free models (big-pickle, gpt-5-nano, etc.) define envVars: [] and
isFree: true but validateAgentApiKey always required at least one
provider key. now the validation is model-aware: free models bypass
the check, keyed models validate their specific vars, and auto-select
still requires at least one known key.

closes #483

Made-with: Cursor

main.ts

@@ -151,6 +151,7 @@ export async function main(): Promise<MainResult> {
 
     validateAgentApiKey({
       agent,
+      model: payload.model,
       owner: runContext.repo.owner,
       name: runContext.repo.name,
     });