fix(cli): improve config init and config list UX

Summary

The frontier config subcommands have UX issues that make it unclear what's happening:

1. frontier config init gives no feedback

Running frontier config init creates a file at ~/.config/frontier.yaml but prints nothing to stdout. It should confirm the file was created and show the path.

Expected behavior:

Config file created at ~/.config/frontier.yaml

2. frontier config list shows empty {}

After running config init, frontier config list outputs just {} — even though a config file exists. It should either display the config values from the file or indicate that defaults are being used.

Current:

$ frontier config list
{}

Expected: Show the actual config values or a helpful message.

🤖 Generated with Claude Code

Start with the frontier config init and frontier config list command entry points and reproduce both behaviors. The work is done when init confirms creation with the config path and list shows the file's values or clearly explains that defaults are being used.