fix(500): the hooks decode a \u escape to UTF-8 in any locale and any awk

scribe_json_unescape wrote each escaped code point with sprintf("%c", n),
which writes the character only under gawk in a UTF-8 locale; gawk in the C
locale and mawk in any locale write the single byte n % 256. The server
escapes every non-ASCII character, so "·" reached the session as 0xB7, "—"
as 0x14 and an emoji as a NUL wherever a hook ran without a UTF-8 locale.
The decoder now encodes UTF-8 itself under LC_ALL=C. A new test sends what
the server sends from a bare environment, in three locales (#5495).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_hook_json_reader.py

@@ -21,6 +21,7 @@ guard here can fail (rule 167).
 from __future__ import annotations
 
 import json
+import os
 import re
 import subprocess
 from pathlib import Path
@@ -156,6 +157,33 @@ def test_a_value_survives_the_round_trip_the_hooks_actually_make(value):
     assert got.rstrip("\n") == value.rstrip("\n")
 
 
+ESCAPED = ["a · b", "an em dash — here", "a face 😀 here", "plain ascii"]
+
+
+@pytest.mark.parametrize("locale", [None, "C", "C.UTF-8"])
+@pytest.mark.parametrize("value", ESCAPED)
+def test_an_escaped_character_decodes_to_utf8_in_any_locale(value, locale):
+    """The server's JSON escapes every non-ASCII character (`\\u00b7`), and a
+    hook may run with no locale at all. The round trip above sends raw UTF-8
+    and never reaches the escape path; this one sends what the server sends,
+    from an environment with nothing but PATH."""
+    need_tools("bash", "awk")
+    env = {"PATH": os.environ["PATH"]}
+    if locale:
+        env["LC_ALL"] = locale
+    body = json.dumps(value)[1:-1]
+    r = subprocess.run(["bash", "-c", f'. "{DEFS}"; scribe_json_unescape'],
+                       input=body.encode(), capture_output=True, env=env)
+    assert r.returncode == 0, r.stderr.decode()
+    assert r.stdout == (value + "\n").encode()
+
+
+def test_the_utf8_guard_can_fail():
+    """A lone byte where the encoded character belongs is what the old
+    decoder wrote; the comparison above has to tell them apart."""
+    assert "·\n".encode() != b"\xb7\n"
+
+
 # --------------------------------------------------------------------------
 # Reading: the shell side.