e2e: register cursor-cli concurrency gate (PR #1577 review)

E2E_CONCURRENT_TEST_LIMIT=2 for cursor-cli was a no-op: AcquireSlot only throttles agents that called RegisterGate, and cursor-cli never did, so its tests ran at full go-test parallelism and kept hitting "Increase limits for faster responses". Register the gate (default 2) so the throttle actually applies.

Note: factoryai-droid has the same latent no-op (sets the env limit but registers no gate) — left as-is pending a decision on whether to actually serialize it.