List a repository's sessions from the terminal and from agents #752

Add a list_repo_sessions tool to kcap mcp sessions and a kcap sessions command over the server's GET /api/repositories/{hash}/sessions (visibility- and access-level-gated, running sessions first). Server half: kcap-server AI-2430, merged as kurrent-io/kcap-server#1810. Spec: kcap-server docs/superpowers/specs/2026-09-02-ai2430-repo-sessions-listing-design.md; plan: kcap-server docs/superpowers/plans/2026-09-02-ai2430-repo-sessions-listing-cli.md.