docs: document GitHub repo creation flow

[rogerchappel]

Summary

• Adds a dedicated GitHub repository creation guide under docs/.
• Documents the explicit --github-create request flow, dry-run-first behavior, --github-execute, and public/private visibility options.
• Captures safety defaults: no-op when not requested, no execute with --dry-run, and gh CLI auth is used only when execution is explicitly requested.

Verification

• pnpm check
• pnpm build
• pnpm check:templates
• pnpm smoke:init
• bash tests/cli-ux-smoke.sh
• Manual GitHub flow smoke for --github-create, --github-visibility public, --github-execute without --github-create, and --github-create --github-execute --dry-run

Notes

The code implementation for the explicit GitHub creation flow is already present on origin/main via merged PR #3. This PR keeps the remaining branch delta scoped to allowed docs/** content.