Fix safe-outputs artifact 409 conflict by using a dedicated artifact name (#21840)

* Initial plan

* Fix safe-outputs artifact upload 409 conflict by using unique artifact name

The safe_outputs job was uploading the safe-output-items.jsonl manifest to
an artifact named "agent", which is the same name used by the main agent
job. GitHub Actions v4 doesn't allow two jobs to upload to the same artifact
name in a workflow run, causing a 409 Conflict error.

Fix: use a dedicated "safe-output-items" artifact name for the manifest
upload instead of "agent".

Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: pelikhan <4175913+pelikhan@users.noreply.github.com>
Co-authored-by: Peli de Halleux <pelikhan@users.noreply.github.com>

Author: Copilot