```yaml
version: "1.0"
updated: 2026-06-21

when: invoked by explicit trigger ("write an agent for X", "create a subagent that does Y", "add an agent to the Z plugin") or implicit request to author a Claude Code subagent or GitHub Copilot CLI plugin agent definition file

# source: omitted — self-authored original; no upstream content adopted

references:
  - https://code.claude.com/docs/en/sub-agents
  - https://code.claude.com/docs/en/plugins-reference
  - https://docs.github.com/en/copilot/reference/custom-agents-configuration
```