Investigate cursor auth-check timeout in CI

[ignatov]

On April 1, 2026, the scheduled Update Agent Versions workflow timed out while running the Verify agent auth support step.

Run: https://github.com/agentclientprotocol/registry/actions/runs/23835002401

Observed behavior:

• check-versions succeeded.
• apply-updates applied version bumps and passed Validate registry build.
• Verify agent auth support stalled on cursor at [9/25] cursor (binary) right after starting the Linux binary download from https://downloads.cursor.com/.../agent-cli-package.tar.gz.
• The workflow hit the step-level 15 minute timeout before the verifier moved on to the next agent.

Relevant log snippet:

• 2026-04-01T06:22:40Z [9/25] cursor (binary)
• 2026-04-01T06:22:40Z -> Downloading from: https://downloads.cursor.com/lab/2026.03.30-a5d3e17/linux/x64/agent-cli-package.tar.gz...
• 2026-04-01T06:37:27Z The action \Verify agent auth support` has timed out after 15 minutes.`

Temporary mitigation:

• cursor has been added to quarantine.json so automated version updates and auth verification skip it until this is understood.

Things to investigate:

• Whether the Cursor download endpoint is intermittently hanging or rate-limiting GitHub Actions runners.
• Whether verify_agents.py needs a stricter per-agent download timeout/retry strategy for binary agents.
• Whether Cursor should stay quarantined permanently or be handled by a narrower workflow path.