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>

README.md

@@ -42,8 +42,17 @@ relspec convert --from pgsql --from-conn "postgres://..." --to sqlite --to-path
 
 # Multiple input files merged
 relspec convert --from json --from-list "a.json,b.json" --to yaml --to-path merged.yaml
+
+# Watch mode: regenerate whenever the source file(s) change (Ctrl-C to stop)
+relspec convert --from dbml --from-path schema.dbml --to gorm --to-path models/ --package models --watch
 ```
 
+`--watch` works with `--from-path` and `--from-list` (not live database
+connections or `--dry-run`). Source files are polled every `--watch-interval`
+(default 500ms), a directory source is watched recursively, and the output path
+is ignored so generating into the source tree does not loop. Conversion errors
+are printed and watching continues.
+
 PostgreSQL connections opened by relspec set `application_name` by default to
 `relspecgo/<version>` (with component suffixes internally, e.g. readers/writers).
 If you need a custom value, provide `application_name` explicitly in the connection