test: harden adapter coverage across CLI and platform edge cases

[vincentkoc]

Summary

• enable Gemini ACP mode in the built-in registry and pin it with a regression test
• add integration coverage for config-defined agent commands with flags and persisted protocol negotiation
• add focused spawn-option coverage for agent, terminal, and queue-owner paths used across tmux/CLI and Windows-style environments

Testing

• pnpm run test

Fixes #44
Refs #48
Refs #64
Refs #67