chore(docs): update pi-fff install command (#418)

* chore(pi-fff): update install command

Document npm installation for the pi extension pi install npm:@ff-labs/pi-fff

* chore: Update docs for - chore(pi-fff): update install command

Author: SamuelLHuber

README.md

@@ -66,7 +66,7 @@ The MCP server gives any agent a file search tool that is faster and more token-
 ### Install
 
 ```bash
-pi extension add @ff-labs/pi-fff
+pi install npm:@ff-labs/pi-fff
 ```
 
 ### Modes

doc/fff.nvim.txt

@@ -72,7 +72,7 @@ Pi agent extension ~
 INSTALL ~
 
 >bash
-    pi extension add @ff-labs/pi-fff
+    pi install npm:@ff-labs/pi-fff
 <