fix(config): apply nested project defaults

[huytdps13400]

Summary

• let Joi synthesize missing iOS and Android project objects so their nested defaults are applied
• restore the documented automaticPodsInstallation: true behavior when project.ios is omitted while preserving an explicit false
• update the stale documentation claim that init creates react-native.config.js

Fixes #2825

Test Plan

• RED: yarn test packages/cli-config/src/__tests__/schema-test.ts --runInBand failed before the schema change because both platform configs resolved to empty objects
• yarn test packages/cli-config/src/__tests__/schema-test.ts --runInBand (5 passed)
• yarn test packages/cli-config/src/__tests__ --runInBand (23 passed)
• yarn test:ci:unit (56 suites, 318 passed, 1 pre-existing todo)
• yarn lint (0 errors; 2 pre-existing warnings outside this diff)
• yarn build
• git diff --check

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout. This schema-only regression is covered directly at the validation boundary instead.