fix: check if compdef exists before registering zsh completion

Fix error "command not found: compdef" when sourcing wt shellenv
in zsh before compinit has been called.

The zsh completion system (compdef) is only available after compinit
has been initialized. Wrap the compdef call in a check to see if the
function exists before trying to use it.

This allows users to safely add 'source <(wt shellenv)' anywhere in
their ~/.zshrc without worrying about the order of initialization.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: timvw

main.go

@@ -477,7 +477,10 @@ if [ -n "$ZSH_VERSION" ]; then
             esac
         fi
     }
-    compdef _wt_complete_zsh wt
+    # Only set up completion if compdef is available (after compinit)
+    if (( $+functions[compdef] )); then
+        compdef _wt_complete_zsh wt
+    fi
 fi
 `)
 	},