fix: modernize dependency recovery guidance (#5282)

docs/guides/configure-langfuse-observability.md

@@ -27,7 +27,7 @@ nanobot agent -m "Hello!"
 Install Langfuse:
 
 ```bash
-python -m pip install langfuse
+nanobot plugins enable langfuse
 ```
 
 ## Minimal working example

docs/provider-cookbook.md

@@ -549,7 +549,7 @@ This recipe applies after the agent works and you want observability for OpenAI-
 Install the optional package in the same Python environment that runs nanobot:
 
 ```bash
-python -m pip install langfuse
+nanobot plugins enable langfuse
 ```
 
 Set the environment variables before starting nanobot: