chore: remove shepherd-task JSON session files flagged by secret scanning

The raw JSON copilot session exports contain API response data that
triggers GitHub secret scanning alerts. The .md session exports provide
equivalent diagnostic value for post-mortem analysis.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 7ae62918-301f-447f-8257-b04f2344263b

Author: edburns