Rationalize config file locations and update documentation

internal/app/plan.go

@@ -19,7 +19,7 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 	var pipelinePath string
 	var sessionPath string
 	var force bool
-	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml")
+	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
 	fs.BoolVar(&force, "force", false, "force stage execution (reserved for future behavior)")
 
@@ -29,11 +29,16 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 	if fs.NArg() != 0 {
 		return fmt.Errorf("plan: unexpected positional arguments")
 	}
-	if pipelinePath == "" || sessionPath == "" {
-		return fmt.Errorf("plan: --config and --session are required")
+	if sessionPath == "" {
+		return fmt.Errorf("plan: --session is required")
 	}
 
-	cfg, err := config.Load(pipelinePath, sessionPath)
+	resolvedPipelinePath, err := resolvePipelineConfigPath(pipelinePath)
+	if err != nil {
+		return fmt.Errorf("plan: %w", err)
+	}
+
+	cfg, err := config.Load(resolvedPipelinePath, sessionPath)
 	if err != nil {
 		return fmt.Errorf("plan: %w", err)
 	}