Final-diff review findings (/simplify-code on the full 3-commit stack):
- Temp-dir fallback uses a per-uid name (hermes-profile-exports-<uid>) and
get_profile_export_path refuses a pre-existing symlink or a directory
owned by another user — a fixed /tmp/hermes-profile-exports is a
predictable shared path a local attacker could pre-create to receive the
secret-bearing archive. Regression test mutation-checked.
- Fail-closed message reworded interface-neutrally (the web API surfaces it
as HTTP 400 detail where '-o' alone made no sense).
- Docs now cover the temp fallback and the fail-closed refusal.
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