feat(cli): add --dry-run to convert, merge and split (#41)

Co-authored-by: SG Command <sgcommand@warky.dev>

cmd/relspec/merge.go

@@ -61,6 +61,7 @@ var (
 	mergeReportPath    string // Path to write merge report
 	mergeFullDDL       bool   // Execute full DDL instead of diffing the live pgsql database
 	mergeFlattenSchema bool
+	mergeDryRun        bool
 )
 
 var mergeCmd = &cobra.Command{
@@ -130,6 +131,7 @@ func init() {
 	mergeCmd.Flags().BoolVar(&mergeVerbose, "verbose", false, "Show verbose output")
 	mergeCmd.Flags().BoolVar(&mergeFullDDL, "full-ddl", false, "pgsql database output: execute the full DDL instead of only the differences from the live database")
 	mergeCmd.Flags().StringVar(&mergeReportPath, "merge-report", "", "Path to write merge report (JSON format)")
+	mergeCmd.Flags().BoolVar(&mergeDryRun, "dry-run", false, "Read and merge in memory, then print the merge plan without writing any output")
 	mergeCmd.Flags().BoolVar(&mergeFlattenSchema, "flatten-schema", false, "Flatten schema.table names to schema_table (useful for databases like SQLite that do not support schemas)")
 }
 
@@ -264,6 +266,29 @@ func runMerge(cmd *cobra.Command, args []string) error {
 			merge.GetColumnTypeConflictSummary(result, 10))
 	}
 
+	if mergeDryRun {
+		if !isMergeOutputFormat(mergeOutputType) {
+			return fmt.Errorf("dry run validation failed: Output: unsupported format '%s'", mergeOutputType)
+		}
+		out := outWriter(cmd)
+		fmt.Fprintf(out, "RelSpec merge plan (dry run - nothing written):\n")
+		fmt.Fprintf(out, "  Target: %s database '%s'\n", mergeTargetType, targetDB.Name)
+		fmt.Fprintf(out, "  Source: %s database '%s'\n", mergeSourceType, sourceDB.Name)
+		switch {
+		case mergeOutputPath != "":
+			fmt.Fprintf(out, "  Output: %s -> %s\n", mergeOutputType, mergeOutputPath)
+		case mergeOutputConn != "":
+			fmt.Fprintf(out, "  Output: %s -> %s\n", mergeOutputType, maskPassword(mergeOutputConn))
+		default:
+			fmt.Fprintf(out, "  Output: %s\n", mergeOutputType)
+		}
+		fmt.Fprintf(out, "  Result:\n")
+		printDryRunPlan(out, targetDB)
+		fmt.Fprintf(out, "\n%s\n", merge.GetMergeSummary(result))
+		fmt.Fprintf(os.Stderr, "\n=== Dry run complete: no output written ===\n")
+		return nil
+	}
+
 	// Step 4: Write output
 	fmt.Fprintf(os.Stderr, "\n[4/4] Writing output...\n")
 	fmt.Fprintf(os.Stderr, "      Format: %s\n", mergeOutputType)
@@ -285,6 +310,16 @@ func runMerge(cmd *cobra.Command, args []string) error {
 	return nil
 }
 
+// isMergeOutputFormat reports whether writeDatabaseForMerge supports dbType.
+func isMergeOutputFormat(dbType string) bool {
+	switch strings.ToLower(dbType) {
+	case "dbml", "dctx", "drawdb", "graphql", "json", "yaml", "gorm", "bun",
+		"drizzle", "prisma", "typeorm", "sqlite", "sqlite3", "pgsql":
+		return true
+	}
+	return false
+}
+
 func readDatabaseForMerge(dbType, filePath, connString, label string) (*models.Database, error) {
 	var reader readers.Reader