fix: add snowballstemmer dep and use --version for 0.20.6

Upstream 0.20.6 added snowballstemmer==3.1.1 (tool_search BM25
stemming) as a direct dependency; package.nix now lists it so
pythonRuntimeDepsCheck passes.

The same release removed the 'hermes version' subcommand; the
package-contents check now uses 'hermes --version' which still
prints 'Hermes Agent'.

Files: package.nix (add snowballstemmer), checks.nix (--version).

package.nix

@@ -223,6 +223,8 @@ pythonPackages.buildPythonApplication {
     firecrawl-py
     fal-client
     parallel-web
+    # tool_search BM25 stemming (upstream >=0.20.6, snowballstemmer==3.1.1)
+    snowballstemmer
     # TTS
     edge-tts
     faster-whisper