[codex] Add one-line guided installer

[DannyDesert]

Summary

• add root install.sh for the documented one-line GitHub install path
• gate deployment behind yclaw doctor so failed credentials/config stop before containers start
• pass GitHub readiness env into non-interactive init and generate GITHUB_WEBHOOK_SECRET
• update README/quickstart docs and add installer/env regression tests

Verification

• npx vitest run packages/cli/tests/generators.test.ts packages/cli/tests/init.test.ts packages/cli/tests/install-script.test.ts
• ./install.sh --help
• bash -n install.sh
• npm run build --workspace=packages/cli
• npm run typecheck
• npm run validate-configs --workspace=packages/core
• npm test
• npm run build