Restore helpful comments to shakapacker.yml

- Restore all useful comments explaining configuration options
- Use YAML anchor syntax (&default, <<: *default) for cleaner inheritance
- Add comment explaining SWC transpiler usage
- Improves developer experience and maintainability

The migration task had removed these comments, but they provide valuable
context for developers working with the configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

spec/dummy/config/shakapacker.yml

@@ -1,66 +1,74 @@
----
-default:
+# Note: You must restart bin/shakapacker-dev-server for changes to take effect
+
+default: &default
   source_path: client/app
   source_entry_path: packs
   public_root_path: public
   cache_path: tmp/cache/shakapacker
   webpack_compile_output: false
   ensure_consistent_versioning: true
-  additional_paths: &1 []
+
+  # Additional paths webpack should lookup modules
+  # ['app/assets', 'engine/foo/app/assets']
+  additional_paths: []
+
+  # Reload manifest.json on all requests so we reload latest compiled packs
   cache_manifest: false
   nested_entries: true
+
+  # Use SWC instead of Babel for transpilation (~20x faster)
   swc: true
+
 development:
-  source_path: client/app
-  source_entry_path: packs
-  public_root_path: public
-  cache_path: tmp/cache/shakapacker
-  webpack_compile_output: false
-  ensure_consistent_versioning: true
-  additional_paths: *1
-  cache_manifest: false
-  nested_entries: true
+  <<: *default
+  # Turn this to true if you want to use the rails/shakapacker check that the test
+  # bundles need building. Also, remove the customization to spec/rails_helper.rb.
   compile: false
+
   public_output_path: webpack/development
+
+  # Reference: https://webpack.js.org/configuration/dev-server/
   dev_server:
     https: false
     host: localhost
     port: 3035
+    # Hot Module Replacement updates modules while the application is running without a full reload
     hmr: true
+    # Defaults to the inverse of hmr. Uncomment to manually set this.
+    # live_reload: true
     client:
+      # Should we show a full-screen overlay in the browser when there are compiler errors or warnings?
       overlay: true
+      # May also be a string
+      # webSocketURL:
+      #  hostname: "0.0.0.0"
+      #  pathname: "/ws"
+      #  port: 8080
+    # Should we use gzip compression?
     compress: true
+    # Note that apps that do not check the host are vulnerable to DNS rebinding attacks
     allowed_hosts: all
     pretty: true
     headers:
       Access-Control-Allow-Origin: "*"
     static:
       watch:
         ignored: "**/node_modules/**"
-  swc: true
+
 test:
-  source_path: client/app
-  source_entry_path: packs
-  public_root_path: public
-  cache_path: tmp/cache/shakapacker
-  webpack_compile_output: false
-  ensure_consistent_versioning: true
-  additional_paths: *1
-  cache_manifest: false
-  nested_entries: true
+  <<: *default
   compile: false
+
+  # Compile test packs to a separate directory
   public_output_path: webpack/test
-  swc: true
+
 production:
-  source_path: client/app
-  source_entry_path: packs
-  public_root_path: public
-  cache_path: tmp/cache/shakapacker
-  webpack_compile_output: false
-  ensure_consistent_versioning: true
-  additional_paths: *1
-  cache_manifest: true
-  nested_entries: true
+  <<: *default
+
   public_output_path: webpack/production
+
+  # Production depends on precompilation of packs prior to booting for performance.
   compile: false
-  swc: true
+
+  # Cache manifest.json for performance
+  cache_manifest: true