Align dotfiles with current tooling

Remove stray .DS_Store, keep Gemfile.lock tracked, and align
dotfile references with the current spec layout and tool versions.

Author: evolve75

.gitignore

@@ -1,4 +1,3 @@
-.DS_Store
 TAGS
 .yardoc/
 coverage.info
@@ -10,4 +9,3 @@ tmp/
 .bundle/
 vendor/
 .idea/
-Gemfile.lock

.readthedocs.yaml

@@ -9,9 +9,9 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    ruby: "3.3"    
+    ruby: "3.1"
   commands:
-    - gem install bundler -v 2.3
+    - gem install bundler -v 2.6.9
     - bundle install
     - bundle exec rake doc:yard
     - mkdir -p $READTHEDOCS_OUTPUT/html

.rubocop_todo.yml

@@ -41,5 +41,6 @@ Metrics/PerceivedComplexity:
 Security/MarshalLoad:
   Exclude:
     - 'lib/tree.rb'
-    - 'spec/tree_spec.rb'
+    - 'spec/tree_initialize_spec.rb'
+    - 'spec/tree_detached_copy_spec.rb'
     - 'test/test_tree.rb'