e2e: color Slack alert by failed-agent tier (red vs yellow)

Every merge reddened main because the 4 chronically-flaky agents (cursor-cli, copilot-cli, codex, factoryai-droid) share one matrix with the reliable ones, so the Slack page fired on every run and the signal went dead. Classify failed jobs: a reliable agent (claude-code, opencode, gemini-cli, roger-roger) or Windows failing is RED (real regression); only flaky agents failing is YELLOW. Mixed failures are RED but list both tiers, so flaky-agent visibility is preserved. Switched the payload to attachments so the color bar renders.