feat(cli): example config in releases and cdd config init/path/edit #70

Part of #61. Covers feature 7 (example config in the release, a command to generate config).

What

• Add a commented config.example.toml at the repo root showing every key with its default, and examples of Actions. It is the single source:
  • //go:embed it into the binary.
  • List it in .goreleaser.yaml archives.files, so it ships in every release tarball.
  • A test loads it through config.LoadFrom and fails if it is invalid.
• Add cdd config subcommands:
  • cdd config init writes the example to the config path, creating directories. It refuses if a file exists, unless given --force.
  • cdd config path prints the config path (respects XDG_CONFIG_HOME).
  • cdd config edit opens the config in $VISUAL/$EDITOR/vi, running init first if the file is missing.
• config joins the Wrapper passthrough list.

Acceptance

• The validity test runs in CI's go test ./....
• init with and without an existing file, and with --force, is tested against a temp XDG_CONFIG_HOME.
• A goreleaser release --snapshot --clean tarball contains config.example.toml.
• README points to cdd config init.