chore(skills): add create-workspace skill for topic jj workspaces

[NathanFlurry]

• Add a create-workspace skill: on "create a new workspace", creates ~/workspaces/{topic}/{agentos,secure-exec} as sibling jj workspaces, fetches main, and starts each on a fresh change off main@origin.
• The skill is strict that the new workspaces become the session's only working directories — the main ~/agent-os / ~/secure-exec checkouts must not be edited afterwards (shared live working copies).