Assign a workflow phase

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: singularity-flow session current --json → ready/workId, cwd=repositoryPath; use CLI/workItemRoot paths; never $HOME.

1. Require an explicit phase and assignee; never infer either from chat identity or the active agent.
2. Inspect current status and authority with singularity-flow status --json before changing the assignment.
3. Run singularity-flow assign <PHASE> <ASSIGNEE> with the exact values supplied.
4. Report the durable assignment result and next action. Assignment coordinates work; it never grants approval authority or changes the governed phase agent.