Show the next Epic action

Output contract: Relay requested CLI fields or output faithfully; preserve warnings/errors and only the explanations required below. For suggested actions, pair Shell with the returned Copilot command; honor commandGuidance. If absent, say "Copilot: no verified equivalent"; never invent a slash command.

Boundary: no Story required; cwd=opened Git root or verified repositoryPath from singularity-flow workspace current --json; refuse if neither resolves; never search $HOME/parents.

1. Resolve the Epic key from the argument or current branch.
2. Run singularity-flow epic next <EPIC-KEY> --json.
3. Present the current phase, blockers, and next action in plain language.
4. Include the exact /sf-* command the user can run next.
5. Do not mutate state or substitute the work-item-only /sf-next.