Rationalize config file locations and update documentation

internal/config/defaults.go

@@ -0,0 +1,18 @@
+package config
+
+// Default filesystem locations for pipeline configuration lookup when --config
+// is omitted. Order is highest to lowest precedence.
+const (
+	DefaultPipelineConfigPathUsrLocal = "/usr/local/etc/narratio/pipeline.yml"
+	DefaultPipelineConfigPathEtc      = "/etc/narratio/pipeline.yml"
+)
+
+// DefaultPipelineConfigSearchPaths defines the default search order for
+// pipeline.yml when callers do not provide an explicit path.
+//
+// Keep this in a variable so future defaults can be extended without changing
+// call sites.
+var DefaultPipelineConfigSearchPaths = []string{
+	DefaultPipelineConfigPathUsrLocal,
+	DefaultPipelineConfigPathEtc,
+}