fix(scripts): exempt .env template files from diff-coverage gate

.env.example and siblings are config/template files with no executable
code. The coverage:diff script now matches them via the same dotfile
pattern used for .gitignore and .npmrc.

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

scripts/coverage/diff.mjs

@@ -47,6 +47,7 @@ const ALLOWED_GLOBS = [
   /\.ya?ml$/,
   /\.gitignore$/,
   /\.npmrc$/,
+  /(^|\/)\.env(\.[^/]+)?$/, // .env, .env.example, .env.local, etc.
   // Shell scripts (not Vitest-covered)
   /\.sh$/,
   /\.bash$/,