fix(sandbox): raise Linux child process limit

[ryana]

Summary

• Raise the sandbox child RLIMIT_NPROC on Linux from 512 to 8192.
• Keep the non-Linux test/default limit at 512 to avoid macOS user-limit failures.
• This fixes AgentHub GLAMR Codex/NeMo Flow failures where Tokio could not spawn threads and the gateway never became ready under Kubernetes.

Validation

• cargo fmt --check
• cargo test -p openshell-sandbox harden_child_process -- --nocapture
• cargo test -p openshell-sandbox process::tests -- --nocapture

AgentHub published test image:

• nvcr.io/nvidian/nemo-llm/agenthub-openshell-supervisor:ghcr-dev-23b66e78
• sha256:a712394d5798a53b6638458c38707be0b268a544a3354ead33b1a5353a1df14c

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[TaylorMutch]

/ok to test 23b66e7

[derekwaynecarr]

See comment here:

#1390 (comment)