feat(init): print success message and next steps after devbox init

[Lagoja]

Summary

Add a success message to devbox init with next steps

How was it tested?

Run devbox init in a test folder, confirm the message is displayed
Rerun the command, confirm the error message still displays

[Copilot]

Pull request overview

Adds post-devbox init user feedback so users immediately see that initialization succeeded and what to do next.

Changes:

• Print a success line after devbox init completes.
• Print “next steps” guidance (e.g., devbox add, devbox shell) after initialization.

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[Copilot]

runInitCmd can return nil in --auto --dry-run mode after only printing the config, but this handler will still print "Created devbox.json..." and next steps. That output is misleading for dry-run; consider skipping the success message when flags.dryRun is true (or have runInitCmd return whether a file was actually created).

Suggested change

	}
	}
	if flags.dryRun {
	return nil
	}

[Copilot]

os.Getwd() error is ignored here. If it fails, path may become empty and the warning/success messages will be confusing (and it also hides the underlying failure). Please handle the error (e.g., return it or fall back to a safe placeholder).

Suggested change

	path, _ = os.Getwd()
	if wd, werr := os.Getwd(); werr != nil {
	ux.Fwarningf(cmd.ErrOrStderr(), "could not determine working directory: %v", werr)
	path = "<unknown working directory>"
	} else {
	path = wd
	}

[Copilot]

pathArg(args) is computed here but runInitCmd also computes pathArg(args) internally. Consider computing the path once in initCmd and passing it down (or returning it from runInitCmd) to avoid duplicated path-resolution logic.