feat: document the setup command in the complete CLI reference #645

What problem are you trying to solve?

DOCUMENTATION.md is presented as the complete CLI command reference, but its command reference starts with read commands and omits testsprite setup. Users currently have to piece together setup behavior and flags from the README and --help, especially for non-interactive authentication, --skip-if-configured, dry runs, profiles, and agent selection.

Verified against commit a18d37c.

Proposed solution

Add a focused Setup (setup) section to DOCUMENTATION.md and its table of contents. Cover interactive and non-interactive examples, credential-source precedence, idempotent reruns, profiles, agent-install flags, dry-run behavior, and success output.

PR #644 contains the documentation-only implementation.

Alternatives considered

Expanding the README further would make the overview too detailed, while relying only on testsprite setup --help leaves the complete browsable reference incomplete. A dedicated section keeps the README concise and the reference complete.

Scope check

• This request is about the CLI itself (commands, flags, output, ergonomics), not server-side product behavior.
• I have searched existing issues and this is not a duplicate.