feat: add since init subcommand

[outofcoffee]

Add a `since init` subcommand that creates a `since.yaml` config file with embedded examples for branch requirements, hooks, and commit exclusions.

Summary

• Add `init` subcommand to create a `since.yaml` config file with comprehensive documented examples
• Embed the config template into the binary using `//go:embed` to avoid external file dependencies
• Export `DefaultConfigFile` constant from `cfg` package for consistent config filename references
• Add unit tests covering directory output, config content validation, and hook/ignore/requireBranch examples

Implementation details

• The config template (`cmd/templates/since.yaml`) uses `//go:embed` to bake examples into the binary, ensuring the init command works without external file dependencies
• Tests use `strings.Contains` for flexible content validation rather than exact string matching, making them resilient to whitespace and ordering changes in the template
• Exposed `DefaultConfigFile` as a package-level constant in `cfg/config.go` so the init command references the same default filename as the config loader