Move init to 'config init', add config edit/get/set subcommands

The config command group manages the config file:
  config init  - write default config (moved from top-level init)
  config edit  - open the config in $EDITOR (falls back to vi)
  config get   - print a value by dotted YAML path (s3.bucket)
  config set   - set a scalar value by dotted YAML path

get/set operate on the yaml.Node tree so comments and formatting in
the config file are preserved across edits. set creates intermediate
maps as needed.

internal/cli/entry_test.go

@@ -18,7 +18,7 @@ func TestCLIEntry(t *testing.T) {
 	}
 
 	// Verify all subcommands are registered
-	expectedCommands := []string{"init", "snapshot", "store", "restore", "prune", "info", "version", "remote", "database"}
+	expectedCommands := []string{"config", "snapshot", "store", "restore", "prune", "info", "version", "remote", "database"}
 	for _, expected := range expectedCommands {
 		found := false
 		for _, cmd := range cmd.Commands() {