Avoid CSCS compute host-key prompt

[robobryce]

Summary

Automatically accept a previously unseen CSCS compute-node host key during the final SSH hop. This removes the interactive pre-authentication prompt that can outlive sshd's login grace period and close the multiplexed proxy stream. Changed host keys are still rejected.

Validation

Validated against the still-running user allocation 4270667 on nid005529:

• connected through the final compute-node SSH hop
• established all five forwards
• JupyterLab returned HTTP 200 through the forwarded port
• Nsight Systems returned HTTP 200 through the forwarded port
• Nsight Compute returned HTTP 200 through the forwarded port
• bash -n, ShellCheck, and git diff --check pass

The allocation and all three services were healthy; no relaunch is required.

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