fix(sandbox): disable child core dumps

[johntmyers]

Summary

Disable core dumps for sandbox child processes in both process spawn and SSH exec flows, and document and test the new hardening.

Related Issue

Linear: OS-75

Changes

• add shared post-fork hardening to set RLIMIT_CORE=0 on Unix and PR_SET_DUMPABLE=0 on Linux
• apply the hardening in both ProcessHandle::spawn_impl() paths and the SSH enter_netns_and_sandbox() path before Landlock/seccomp enforcement
• add fork-based unit coverage and an e2e regression that asserts ulimit -c is 0 inside the sandbox
• update architecture and security documentation to reflect the startup ordering and guarantee

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Local verification:

• cargo fmt --all
• cargo test -p openshell-sandbox harden_child_process_disables_core_dumps --lib
• mise run python:proto
• mise run pre-commit on this host (z3.h missing for z3-sys)
• mise run test on this host (z3.h missing for z3-sys)
• mise run e2e locally (stopped; planning to rely on GitHub CI)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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