fix: use mcp-scripts for traffic API auth

Replace sandbox.agent.env approach with mcp-scripts tool.
The fetch-traffic MCP script runs outside the sandbox with
GH_AW_GITHUB_TOKEN, bypassing the sandbox's token exclusion.

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

Author: DanWahlin