fix: only re-stage previously staged files in pre-commit hook

Capture staged file list before prettier runs, then re-add only
those files. Prevents pulling in unrelated unstaged changes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.husky/pre-commit

@@ -1,2 +1,5 @@
+staged=$(git diff --cached --name-only --diff-filter=ACM -- 'src/**/*.ts')
 pnpm run format:fix
-git diff --name-only -- 'src/**/*.ts' | xargs git add 2>/dev/null || true
+if [ -n "$staged" ]; then
+  echo "$staged" | xargs git add
+fi