Add bb.edn runner scripts and improve nREPL port handling

- bb.edn with compile/test/repl/nrepl/clean/fmt tasks
- nREPL checks .nrepl-port for existing server before starting
- Cleans up .nrepl-port on shutdown
- Add .nrepl-port to .gitignore

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -8,3 +8,4 @@ erl_crash.dump
 *.beam
 /tmp/
 .elixir_ls/
+.nrepl-port