test(install): add UTF-8 regression guard for skills_sync child path

Addresses hermes-sweeper review on PR #54866: the installer runs
tools/skills_sync.py as a child python.exe whose PYTHONIOENCODING /
PYTHONUTF8 the scoped install.ps1 block sets, but there was no
regression test for this child-Python UTF-8 path. The existing
test_child_process_inherits_utf8_mode covers a different (bootstrap
entry-point) flow.

Add TestSkillsSyncUtf8Guard: three subprocess tests that import
skills_sync (triggering its import-time stdout/stderr reconfigure)
and assert the checkmark/up-arrow glyphs the script prints at
tools/skills_sync.py:596,675 emit valid UTF-8 and exit 0 even when
the child env is left unset or explicitly hostile (gbk). A third
test proves the guard is load-bearing by reproducing the crash
without it.

Also keep the new install.ps1 comment ASCII-only (the checkmark
spelled out as U+2713) per the file's PS 5.1 parser-compatibility
contract at scripts/install.ps1:79-80; the literal glyph in the
comment violated that contract.

scripts/install.ps1

@@ -2438,9 +2438,10 @@ You are Hermes Agent, an intelligent AI assistant created by Nous Research. You
             # Force the child python.exe to emit UTF-8 on its stdout/stderr.
             # On non-UTF-8 Windows locales (CP936/GBK zh-CN) Python defaults
             # its stream encoding to the active codepage and crashes on glyphs
-            # like ✓ (U+2713) that the codepage can't encode; the resulting
-            # non-UTF-8 bytes break this script's JSON result frame on stdout
-            # and abort the config-templates stage. Scope to this call only.
+            # like the checkmark (U+2713) that the codepage can't encode; the
+            # resulting non-UTF-8 bytes break this script's JSON result frame on
+            # stdout and abort the config-templates stage. Scope to this call
+            # only. (Comment kept ASCII per this file's PS 5.1 contract above.)
             $prevPythonioencoding = $env:PYTHONIOENCODING
             $prevPythonutf8 = $env:PYTHONUTF8
             $env:PYTHONIOENCODING = "utf-8"

tests/test_hermes_bootstrap.py

@@ -24,6 +24,7 @@ import os
 import subprocess
 import sys
 import textwrap
+from pathlib import Path
 
 import pytest