feat: enforce files end with single newline via super-linter

Add EditorConfig configuration to ensure all files end with a single
newline character and trim trailing whitespace. Enable VALIDATE_EDITORCONFIG
in super-linter workflow to enforce these rules in CI.

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

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

Author: dhellmann

.editorconfig

@@ -0,0 +1,8 @@
+root = true
+
+[*]
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.md]
+trim_trailing_whitespace = false

.github/workflows/check.yaml

@@ -123,3 +123,4 @@ jobs:
           # https://github.com/super-linter/super-linter/blob/main/docs/run-linter-locally.md#share-environment-variables-between-environments
           VALIDATE_ALL_CODEBASE: false
           VALIDATE_MARKDOWN: true
+          VALIDATE_EDITORCONFIG: true