feat: implement interactive lesson management and update command handling

src/app/mode/help.rs

@@ -23,10 +23,7 @@ Slash commands:
   /help             Show this help
   /quit             Quit session
   /mode <name>      Switch mode (chat, bash, workflow)
-  /lesson <text>    Create a lesson
-  /lesson list      List lessons
-  /lesson export    Export lessons
-  /lesson import    Import lessons
+  /lesson           Interactive lesson manager
   /clear            Clear transcript";
 
 /// Route an incoming action while the help overlay is open.