Draft an EPIC from a GitHub issue and open a PR for review. Use when triggered by a routine on `Issue opened` with label `ai:plan-epic`, or when a user asks to "draft an epic from issue #NNN". Non-interactive — proceeds on best interpretation and surfaces unresolved questions in the EPIC's §Open Questions section rather than blocking. The PR review loop resolves ambiguity. Does NOT write child SPECs — slice SPECs are drafted lazily by the `draft-spec` routine when the human files one `ai:plan` issue per slice after the EPIC PR is merged.