Clean up Jupyter Nsight integration

[robobryce]

Summary

• remove the unused jupyterlab-nvidia-nsight extension from every tutorial image
• remove its obsolete *.brevlab.com/TURN settings and dead BREV_ENV_ID Compose plumbing
• retain theme and execution-timing preferences in a neutral Jupyter settings script
• restart the sibling nsys and ncu Compose services after a clean Jupyter shutdown
• keep external container stops and failed Jupyter starts from cascading restarts

Rationale

The Nsight Systems and Nsight Compute launcher entries are provided by jupyter-server-proxy, not jupyterlab-nvidia-nsight. This was confirmed on a live Brev node through /server-proxy/servers-info; both /nsys/ and /ncu/ returned HTTP 200 independently of the legacy extension UI.

The restart path stays in the root Jupyter wrapper, which already has the Docker socket. Jupyter remains unprivileged. A small standard-library helper identifies sibling containers using Docker Compose labels and restarts only nsys and ncu after Jupyter exits cleanly.

Related: #209 restores the socat forwarders used by the named proxies.

Validation

• live-node launcher/proxy ownership and HTTP checks
• disposable Docker Engine integration test restarted labeled nsys and ncu siblings
• wrapper tests for clean exit, failed exit, and external termination
• generated Jupyter theme/timing settings parse as JSON with no Nsight plugin settings
• docker compose config --quiet for every Brev Compose file
• bash -n, shellcheck, and Python compilation
• repository pre-commit hooks
• lychee validation for the changed documentation
• commit signature hook

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[github-actions]

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[robobryce]

Upstream CI is ready for maintainer approval on signed head f4b2988. Please run /ok to test f4b2988 so I can monitor and address the full image-build and tutorial-test matrix.