Add .editorconfig

Enforces 4-space indentation (tabs for Go and Makefiles), LF line
endings, UTF-8 encoding, and trailing-whitespace trimming.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.editorconfig

@@ -0,0 +1,15 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 4
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[Makefile]
+indent_style = tab
+
+[*.go]
+indent_style = tab