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.
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