fix: lower mkdocs-llmstxt to >=0.4.0 for Python 3.9 compatibility

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

Author: arieradle

pyproject.toml

@@ -99,7 +99,7 @@ dev = [
     "mkdocs-mermaid2-plugin>=1.0.0",
     "mkdocs-minify-plugin>=0.8.0",
     "mkdocs-rss-plugin>=1.17.0",
-    "mkdocs-llmstxt>=0.5.0",
+    "mkdocs-llmstxt>=0.4.0",
     "google-genai>=1.0.0",
     "huggingface-hub>=0.20.0",
     "opentelemetry-api>=1.0.0",