Default bb CLI connection to local server

[SawyerHood]

Summary

• Default the standalone bb CLI connection to the local packaged server when BB_SERVER_URL is unset
• Default the CLI host daemon port to 38887 when BB_HOST_DAEMON_PORT is unset, while preserving source dev repo-root defaults
• Update CLI config docs, package README, and built-in bb-cli skill guidance

Validation

• pnpm exec turbo run test --filter=@bb/config
• pnpm exec turbo run typecheck --filter=@bb/config
• pnpm exec turbo run test --filter=@bb/cli
• pnpm exec turbo run typecheck --filter=@bb/cli