fix: simplify highlights output by writing file directly

Remove custom safe-output job entirely. Instead, the agent writes
highlights to /tmp/gh-aw/release-highlights.md via shell, which is
automatically collected in the agent artifact. The release workflow
downloads the agent artifact to get the file.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lostindark