Add session discovery and template support

internal/app/plan.go

@@ -21,9 +21,11 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 
 	var pipelinePath string
 	var sessionPath string
+	var sessionID string
 	var force bool
 	fs.StringVar(&pipelinePath, "config", "", "path to pipeline.yml (optional; defaults searched)")
 	fs.StringVar(&sessionPath, "session", "", "path to session.yml")
+	fs.StringVar(&sessionID, "session-id", "", "session identifier for session.yml templates")
 	fs.BoolVar(&force, "force", false, "force stage execution (reserved for future behavior)")
 
 	if err := fs.Parse(args); err != nil {
@@ -32,16 +34,18 @@ func Plan(ctx context.Context, args []string, out io.Writer) error {
 	if fs.NArg() != 0 {
 		return fmt.Errorf("plan: unexpected positional arguments")
 	}
-	if sessionPath == "" {
-		return fmt.Errorf("plan: --session is required")
-	}
-
 	resolvedPipelinePath, err := resolvePipelineConfigPath(pipelinePath)
 	if err != nil {
 		return fmt.Errorf("plan: %w", err)
 	}
+	resolvedSessionPath, err := resolveSessionConfigPath(sessionPath)
+	if err != nil {
+		return fmt.Errorf("plan: %w", err)
+	}
 
-	cfg, err := config.Load(resolvedPipelinePath, sessionPath)
+	cfg, err := config.LoadWithSessionOptions(resolvedPipelinePath, resolvedSessionPath, config.SessionLoadOptions{
+		SessionID: sessionID,
+	})
 	if err != nil {
 		return fmt.Errorf("plan: %w", err)
 	}