🔒 fix(fleet): use mktemp for unpredictable temporary report file path

[jackby03]

🎯 What: Replaced predictable timestamp-based temporary file creation (/tmp/hardbox-fleet-%d.json) in internal/fleet/fleet.go with mktemp /tmp/hardbox-fleet-XXXXXX executed on target remote systems via SSH.

⚠️ Risk: Constructing predictable temporary file names in /tmp using timestamps opens a window for symlink attacks, race conditions, or unprivileged local user tampering with audit report files prior to or during writing on the host.

🛡️ Solution: mktemp generates unpredictable temporary file paths with restricted permissions (0600). Deferred file cleanup ensures the report file is removed regardless of audit success or failure.

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PR created automatically by Jules for task 8346812920574847013 started by @jackby03

[google-labs-jules]

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