clear_agent_cache falls back to renaming the agent cache directory
to <cache_dir>/.trash/<agent_id>-<nanos>-<counter>/ when
remove_dir_all fails — typically because a running <cmd>.exe or a
Defender scan holds the file. NTFS rename succeeds on directories
whose children are locked, so Upgrade and Uninstall no longer
surface ERROR_ACCESS_DENIED on Windows.
A detached OS thread sweeps .trash/ at startup with all errors
swallowed, panics caught, and no shared state — cannot block app
startup or leak threads. Still-locked entries are left for the
next launch.
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