Add basectl onboard to README Quick Start

[codeforester]

Context

basectl onboard (added in Base v1.3.0, polished through v1.5.0) orchestrates the full first-time Base setup sequence: check, setup, update-profile, doctor, and project discovery. It is the recommended first-run path for a developer setting up Base for the first time.

The README.md Quick Start section lists individual commands (basectl setup base-demo, basectl check base-demo, etc.) without mentioning basectl onboard. A new developer following the README has no way to discover that a single command covers the entire onboarding sequence.

Scope

• Add a short paragraph to the README.md Quick Start section explaining basectl onboard as the recommended first-run path for new Base users, before the individual command list.
• Mention that basectl onboard --dry-run can preview onboarding steps without making changes.
• Keep the existing individual command list intact as the reference for post-onboarding exploration.
• No demo script or BATS changes needed unless a contract row is added.

Validation

• README.md mentions basectl onboard in the Quick Start section.
• basectl onboard --help output and the README description are consistent.