Research a GitHub issue and produce a detailed implementation plan, posted as an issue comment for a human or a different agent to pick up cold. Runs under Claude Code's plan mode: all research and drafting happens read-only, and the plan is only posted to the issue after the user approves it via ExitPlanMode. Trigger when the user runs /plan-issue with a GitHub issue URL or number (e.g. /plan-issue https://github.com/owner/repo/issues/42 or /plan-issue #42), or says "plan out issue #N", "draft a plan for #N", "write a plan for #N". Language-agnostic. The skill never branches, opens a PR, or writes code -- see /pick-up-issue for execution.
