Make npx usage and provider options explicit

[EmersonBraun]

What

Makes the first-run path explicit and CLI-only:

• replaces clone + npm run onboarding with one working npx command
• adds a provider table inside CLI reference with credentials, model requirements, and examples
• rewrites common commands around the same npx entry point
• adds prepare so GitHub-sourced npx installs build the executable
• stops emitting TypeScript declarations because this package exposes a CLI, not a public library API
• keeps the legacy --api default while requiring explicit models for normal API-provider selection

Verification

• npm run check
• local npx --package . agentskit-review --list-providers
• clean-cache npx github:AgentsKit-io/code-review-cli#codex/npx-readme-clarity --list-providers
• git diff --check