The desktop updater ran `hermes update --yes`, which auto-restored any
uncommitted source-tree edits onto the freshly updated checkout. On dirty
from-source installs this silently carried local modifications across every
update and could break the rebuilt app (field report: Windows update handoff
leaving the app 'crashed').
New `hermes update --keep-stash`: local changes are still autostashed so the
update can proceed, but are never re-applied — they stay parked in git stash
with printed recovery guidance. Both desktop handoff scripts (windows.ps1,
posix.sh) now pass it, probing `update --help` first so older installed
backends without the flag keep working. Failure paths are unchanged (stash
preserved, no restore); updates.non_interactive_local_changes: discard still
wins.
Tests: park/restore/failure-path coverage incl. a sabotage-verified
regression test; docs updated.
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