docs: clarify CommonMarker testing and extension behavior

I keep the development version override in the README and place full test commands in CONTRIBUTING. I correct comments about modern-only extensions without changing rendering.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot-Session: 6fe372b1-3ff6-4129-9d33-be564fb6a5f0
Signed-off-by: Zack Koppert <zkoppert@github.com>

Author: zkoppert

README.md

@@ -84,7 +84,7 @@ cd script
 ./bootstrap
 ```
 
-The development bundle uses CommonMarker 2.8.2 by default. Set `COMMONMARKER_VERSION` to test another version, then resolve dependencies with `bundle update commonmarker`. See [Testing](CONTRIBUTING.md#testing) for commands.
+Set `COMMONMARKER_VERSION` to override the development bundle's default `~> 2.8.2` requirement. See [Testing](CONTRIBUTING.md#testing) for parser version commands.
 
 Contributing
 ------------

lib/github/markup/markdown.rb

@@ -64,17 +64,16 @@ class Markdown < Implementation
                  :greentext, :alerts, :description_lists
               extension_options[ext] = true
             when :header_ids
-              # header_ids takes a string prefix in 2.x rather than a boolean. The legacy contract
-              # only passed it as a symbol, so use an empty prefix to enable anchor generation.
+              # header_ids takes a string prefix in 2.x rather than a boolean. An empty prefix
+              # enables anchor generation for the symbol-based commonmarker_exts option.
               extension_options[:header_ids] = ""
             else
               raise ArgumentError, "unknown commonmarker extension: #{ext.inspect}"
             end
           end
 
-          # Several extensions (tagfilter, autolink, table, strikethrough, tasklist, shortcodes)
-          # are enabled by default in commonmarker 2.x but were strictly opt-in in 0.x. Explicitly
-          # disable any extension the caller did not request so behavior matches the legacy contract.
+          # CommonMarker 2.x enables more extensions by default than this wrapper does.
+          # Disable unrequested defaults to preserve the wrapper's extension choices.
           [:strikethrough, :tagfilter, :autolink, :table, :tasklist, :shortcodes].each do |ext|
             extension_options[ext] = false unless extension_options[ext]
           end