Hide non-primary agents from the Tab switcher

[dmoliveira]

Summary

• generate hidden metadata for custom secondary agents so only build, plan, and orchestrator are intended Tab-switcher primaries
• document and validate the hidden-subagent contract in agent docs and agent_doctor
• add a config override for external loop agents so executor, planner, supervisor, oracle, and reviewer stay hidden in the live runtime

Validation

• python3 scripts/build_agents.py --profile balanced --check
• python3 scripts/agent_doctor.py --json in an isolated temp install
• opencode debug config confirmed the effective visible primary set is limited to build, plan, and orchestrator