test(opencode): remove flaky subagent test

[opencode-agent]

Summary

• remove the concurrent nested-subagent permission subprocess test
• retain the simple production fix that routes child-session permission requests
• stop the test’s hard 10-second subprocess timeout from failing Linux CI under suite contention

Testing

• bun test test/cli/run/run-process.test.ts --test-name-pattern "rejects requested permissions by default"
• bun x prettier --check test/cli/run/run-process.test.ts

Requested by: @rekram1-node (Aiden via Slack)