feat(cli): add --watch mode to convert (#39)

Poll source files/directories and regenerate output on change.
Output path is excluded from watching; errors don't stop the loop.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

cmd/relspec/convert.go

@@ -59,6 +59,8 @@ var (
 	convertContinueOnError bool
 	convertExtraFields     string
 	convertDryRun          bool
+	convertWatch           bool
+	convertWatchInterval   time.Duration
 )
 
 var convertCmd = &cobra.Command{
@@ -167,7 +169,11 @@ Examples:
 
   # Convert SQLite to PostgreSQL SQL
   relspec convert --from sqlite --from-path database.db \
-                  --to pgsql --to-path schema.sql`,
+                  --to pgsql --to-path schema.sql
+
+  # Regenerate GORM models every time the DBML file changes
+  relspec convert --from dbml --from-path schema.dbml \
+                  --to gorm --to-path models/ --package models --watch`,
 	RunE: runConvert,
 }
 
@@ -189,6 +195,9 @@ func init() {
 
 	convertCmd.Flags().BoolVar(&convertDryRun, "dry-run", false, "Read and validate the input and print the plan without writing any output")
 
+	convertCmd.Flags().BoolVar(&convertWatch, "watch", false, "Watch the source files (--from-path or --from-list) and regenerate the output whenever they change")
+	convertCmd.Flags().DurationVar(&convertWatchInterval, "watch-interval", 500*time.Millisecond, "Polling interval used by --watch")
+
 	err := convertCmd.MarkFlagRequired("from")
 	if err != nil {
 		fmt.Fprintf(os.Stderr, "Error marking from flag as required: %v\n", err)
@@ -204,6 +213,13 @@ func init() {
 }
 
 func runConvert(cmd *cobra.Command, args []string) error {
+	if convertWatch {
+		return runConvertWatch(cmd.Context(), os.Stderr, func() error { return runConvertOnce(cmd) })
+	}
+	return runConvertOnce(cmd)
+}
+
+func runConvertOnce(cmd *cobra.Command) error {
 	fmt.Fprintf(os.Stderr, "\n=== RelSpec Schema Converter ===\n")
 	fmt.Fprintf(os.Stderr, "Started at: %s\n\n", getCurrentTimestamp())