Prevent recursion when invoking AI

This can result in stack overflow errors for missing commands.

https://fizzy.37signals.com/5986089/collections/7/cards/1135

Follow up to https://github.com/basecamp/fizzy/pull/766

app/models/command/parser.rb

@@ -3,8 +3,9 @@ class Command::Parser
 
   delegate :user, :cards, :filter, :script_name, to: :context
 
-  def initialize(context)
+  def initialize(context, fall_back_to_ai: true)
     @context = context
+    @fall_back_to_ai = fall_back_to_ai
   end
 
   def parse(string)
@@ -17,6 +18,10 @@ class Command::Parser
   end
 
   private
+    def fall_back_to_ai?
+      @fall_back_to_ai
+    end
+
     def as_plain_text(string)
       ActionText::Content.new(string).to_plain_text
     end
@@ -100,7 +105,7 @@ class Command::Parser
       elsif card = single_card_from(string)
         Command::GoToCard.new(card_id: card.id)
       else
-        Command::Ai::Parser.new(context).parse(string)
+        parse_with_fallback(string)
       end
     end
 
@@ -114,4 +119,12 @@ class Command::Parser
     def single_card_from(string)
       user.accessible_cards.find_by_id(string)
     end
+
+    def parse_with_fallback(string)
+      if fall_back_to_ai?
+        Command::Ai::Parser.new(context).parse(string)
+      else
+        Command::Search.new(terms: string)
+      end
+    end
 end