add pass_filenames_via_stdin for large changesets

pre-commit currently passes selected filenames to hooks via argv.
For large changesets (or --all-files), argv length limits are hit and
filenames are partitioned, causing multiple hook invocations.

This means there is currently no built-in way to pass filenames to an
underlying hook in one shot without chunking / re-running. The only practical
workaround is to set pass_filenames: false and run custom git operations in
hook code to reconstruct the file set, which is expensive and duplicates
pre-commit's own file-selection logic.

This change adds a hook option:

    pass_filenames_via_stdin: true

When enabled, pre-commit sends filenames as NUL-delimited bytes on stdin and
runs the hook in a single invocation (no argv chunking).

Why NUL-delimited stdin:
- safe for filenames containing spaces/newlines
- matches established -0 conventions in unix tooling

Usage for hook authors:
- shell:

    while IFS= read -r -d '' filename; do
        ...
    done

- python:

    data = sys.stdin.buffer.read()
    filenames = [os.fsdecode(p) for p in data.split(b'\0') if p]

Behavior notes:
- default remains argv-based passing
- pass_filenames: false still disables filename passing entirely

Implementation includes schema/runtime wiring, shared NUL encode/decode
helpers, and tests covering defaulting and runtime behavior.

tests/languages/pygrep_test.py

@@ -1,5 +1,8 @@
 from __future__ import annotations
 
+import sys
+from unittest import mock
+
 import pytest
 
 from pre_commit.languages import pygrep
@@ -138,6 +141,16 @@ def test_grep_hook_matching(some_files, tmp_path):
     assert ret == (1, b"f7:1:hello'hi\n")
 
 
+@pytest.mark.usefixtures('some_files')
+def test_main_reads_nul_delimited_filenames_from_stdin(cap_out):
+    with mock.patch.object(sys.stdin.buffer, 'read', return_value=b'f1\x00f2\x00'):
+        ret = pygrep.main(('foo',))
+
+    out = cap_out.get()
+    assert ret == 1
+    assert out == 'f1:1:foo\n'
+
+
 @pytest.mark.parametrize('regex', ('nope', "foo'bar", r'^\[INFO\]'))
 def test_grep_hook_not_matching(regex, some_files, tmp_path):
     ret = run_language(tmp_path, pygrep, regex, file_args=('f7', 'f8', 'f9'))