Support loading a yaml configuration file

[yalefresne]

Goal

Support loading a configuration file from the path ~/.config/dvopsctl.yaml in the user's home directory when no config path is explicitly provided.

Context

Currently, the dvopsctl CLI requires an explicit path or defaults to a fixed location. To improve developer experience and follow XDG Base Directory conventions, we want to support loading config from ~/.config/dvopsctl.yaml.

This behavior should only apply if no other config path is passed via CLI flags or environment variables.

Acceptance Criteria

• Implement logic to resolve ~/.config/dvopsctl.yaml using os.UserConfigDir() or similar
• If a path is passed via flag or environment variable, it should override the default
• If no config file is found in any location, return a clear error message
• Write unit tests using testing package that:
  • Mock or simulate the presence of a file at ~/.config/dvopsctl.yaml
  • Assert the file is loaded correctly as default
  • Assert a user-provided path takes priority
  • Assert proper error is returned if no config exists
• Update CLI help text and documentation to describe the default config path behavior

Resources

• Use os.UserHomeDir() or os.UserConfigDir() to resolve user paths
• Use os.Open, ioutil.ReadFile, or os.ReadFile to read YAML
• Consider using t.TempDir() and os.Setenv() for test isolation
• https://specifications.freedesktop.org/basedir-spec/latest/