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/commands/run_test.py

@@ -1064,6 +1064,35 @@ def test_pass_filenames(
     assert (b'foo.py' in printed) == pass_filenames
 
 
+def test_pass_filenames_via_stdin(cap_out, store, repo_with_passing_hook):
+    config = {
+        'repo': 'local',
+        'hooks': [{
+            'id': 'filenames-via-stdin',
+            'name': 'filenames-via-stdin',
+            'entry': (
+                f'{shlex.quote(sys.executable)} -c '
+                '\'import sys; '
+                'print(repr(sys.argv[1:])); '
+                'print(repr(sys.stdin.buffer.read()))\''
+            ),
+            'language': 'system',
+            'pass_filenames_via_stdin': True,
+        }],
+    }
+    add_config_to_repo(repo_with_passing_hook, config)
+    stage_a_file()
+
+    ret, printed = _do_run(
+        cap_out, store, repo_with_passing_hook,
+        run_opts(hook='filenames-via-stdin', verbose=True),
+    )
+
+    assert ret == 0
+    assert b'[]' in printed
+    assert b"b'foo.py\\x00'" in printed
+
+
 def test_fail_fast(cap_out, store, repo_with_failing_hook):
     with modify_config() as config:
         # More than one hook