Synchronize an Epic

Output contract: Let the CLI validate and mutate state; preserve its exact result, warnings, publication status, artifacts, and next actions. 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 sync <EPIC-KEY> --json.
3. Report every fetched repository, branch, commit, stale context, and pending publication.
4. If synchronization cannot fast-forward, stop and show the conflicting repository; never force-push.
5. End with /sf-epic-next <EPIC-KEY>.