add prompt & flags to select repo name in dot mod command

[charlesHetterich]

closes #24

When forking, dot mod now prompts for the repo name after app selection. Enter keeps the auto-generated default; anything else replaces it. Prompt is skipped for --clone (local dir only), -y (non-interactive), and --repo-name <name> (explicit override). Names are validated against GitHub's rules and against existing directories on disk.

Flags added: -y, --yes, --repo-name <name>.

Extra

• Removed the dead --no-install flag from dot mod.
• Replaced the dot mod (stub) block in README.md with a real section.

[github-actions]

Dev build ready — try this branch:

curl -fsSL https://raw.githubusercontent.com/paritytech/playground-cli/main/install.sh | VERSION=dev/mod-choose-repo-name bash