Fix low-severity bugs, stale docs, and repo housekeeping

[tas50]

Summary

A batch of small fixes found while reviewing the project.

Library

• List/hash methods without a block — apple with no block raised NoMethodError: undefined method 'arity' for nil. It now raises ArgumentError: apple requires a block to configure the new apple.
• Case-insensitive extensions — from_file("config.YML") evaluated the YAML as Ruby. Extensions are now matched case-insensitively.
• File.read instead of IO.read — on Ruby 3.x, IO.read runs a shell command when the filename starts with | (for example "|cmd #.yml"). Ruby 4.0 removed this, but the gem still supports 3.1.
• respond_to_missing? — respond_to?(:foo) now returns true for a value set without a configurable, matching what method_missing handles. It stays false for keys that were never set, so duck-typing checks such as to_ary are unaffected.
• Documented default caching — reading an option with a static default stores a copy of that default, so mutable defaults can be modified in place. Afterwards the option appears in save even though it still holds its default. The save doc now says so.
• Dead code and stale comments — removed the unused private to_dotted_hash, replaced writes_with() with config_attr_writer() in the strict mode docs, fixed the save example (Mixlib::Config is a module you extend, not a class to inherit from), reworded the stale "remove this in 3.0" note on hash_dup, and fixed a truncated sentence and a typo.

Specs

• Replaced the deprecated expect(lambda { ... }) form with expect { ... }. The suite printed 15 deprecation warnings before this change and prints none now.
• Specs now stub File.read. Removed stubs of File.exists? (removed in Ruby 3.2) and File.readable?, which from_file never calls.

Repo

• rake style now fails when cookstyle is missing. It used to print a message and exit 0.
• lint.yml: removed a dictionary download nothing used (cspell loads the dictionary from its URL) and a BUNDLE_WITHOUT naming groups that don't exist in this Gemfile.
• Restored the bug report template's emoji, which had been saved as U+FFFD replacement characters.
• CONTRIBUTING.md now links chef/chef's main branch.
• .gitignore no longer ignores the tracked .rspec file, and a duplicate .yardoc entry is gone.
• Fixed typos in the Rakefile and the Expeditor scripts.
• Added vkey and yardopts to the cspell word list. Spellcheck only checks changed files, and both words were already present in files this PR touches.

Merge notes

This PR edits some of the same lines as #140 (the list/hash accessor methods) and #142 (from_yaml/from_json). Whichever merges second will need a small rebase.

Tests

Added specs for the no-block errors, the uppercase extension, and respond_to?. The updated File.read stubs failed against IO.read. The full suite passes (161 examples) with no deprecation warnings, and cookstyle and cspell are clean.