Use idiomatic Ruby booleans

Prefer nil? and direct boolean checks in TreeNode/TrieNode while
preserving existing semantics, and ensure include? returns a boolean.

Author: evolve75

lib/tree.rb

@@ -161,7 +161,7 @@ def root?
     #
     # @return [Boolean] +true+ if the node has content.
     def content?
-      @content != nil
+      !@content.nil?
     end
 
     alias has_content? content? # @todo: Aliased for eventual replacement

lib/tree/trie.rb

@@ -70,7 +70,7 @@ def initialize(name, content = nil, options = nil)
     #
     # @return [Boolean] +true+ if this is a terminal node.
     def terminal?
-      @terminal == true
+      @terminal
     end
 
     # Inserts a word into the trie.
@@ -119,7 +119,7 @@ def <<(word)
     def include?(word)
       validate_word!(word)
       node = find_node(word)
-      node&.terminal? == true
+      !!node&.terminal?
     end
 
     # Alias for {#include?} to support search-oriented naming.
@@ -173,7 +173,7 @@ def words_with_prefix(prefix, limit: nil)
     # @param [Boolean] value The terminal flag value.
     # @return [void]
     def terminal=(value)
-      @terminal = value == true
+      @terminal = value.equal?(true)
     end
 
     # Validate that the word is a non-empty string.