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/clientlib_test.py

@@ -572,6 +572,18 @@ def test_manifest_stages_defaulting():
     ]
 
 
+def test_manifest_pass_filenames_via_stdin_defaulting():
+    dct = {
+        'id': 'fake-hook',
+        'name': 'fake-hook',
+        'entry': 'fake-hook',
+        'language': 'system',
+    }
+    cfgv.validate(dct, MANIFEST_HOOK_DICT)
+    dct = cfgv.apply_defaults(dct, MANIFEST_HOOK_DICT)
+    assert dct['pass_filenames_via_stdin'] is False
+
+
 def test_config_hook_stages_defaulting_missing():
     dct = {'id': 'fake-hook'}
     cfgv.validate(dct, CONFIG_HOOK_DICT)