Add remove-em-dash hook

New fixer hook that replaces UTF-8 em-dashes (U+2014) with a plain
hyphen (-), modeled on the trailing-whitespace hook.

- pre_commit_hooks/remove_em_dash.py: the fixer (binary-safe, UTF-8 only)
- tests/remove_em_dash_test.py: full coverage of fix and no-op cases
- registered in setup.cfg, .pre-commit-hooks.yaml, and README.md

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pre_commit_hooks/remove_em_dash.py

@@ -0,0 +1,34 @@
+from __future__ import annotations
+
+import argparse
+from collections.abc import Sequence
+
+EM_DASH = '\N{EM DASH}'.encode()
+
+
+def _fix_file(filename: str) -> bool:
+    with open(filename, 'rb') as f:
+        contents = f.read()
+    new_contents = contents.replace(EM_DASH, b'-')
+    if new_contents == contents:
+        return False
+    with open(filename, 'wb') as f:
+        f.write(new_contents)
+    return True
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    parser = argparse.ArgumentParser()
+    parser.add_argument('filenames', nargs='*', help='Filenames to fix')
+    args = parser.parse_args(argv)
+
+    retv = 0
+    for filename in args.filenames:
+        if _fix_file(filename):
+            print(f'Fixing {filename}')
+            retv = 1
+    return retv
+
+
+if __name__ == '__main__':
+    raise SystemExit(main())