Add cli-v2 slash command foundation

[roackb2]

Summary

• add a cli-v2 terminal slash command service boundary and README
• support /help, /new [name], /sessions, and /auth guidance without importing old TUI/core command code
• route slash submissions through ControlPlaneSessionStore before agent prompt submission

Verification

• yarn test:unit src/tests/unit/cli-v2/terminal-slash-command-service.test.ts src/tests/unit/cli-v2/control-plane-session-store.test.ts
• yarn -s typecheck