docs: document ptrace_scope requirement for shell hook registration

[moukrea]

Summary

• Document kernel.yama.ptrace_scope=0 as a requirement for shell hook registration
• Add fix instructions (temporary and permanent) to the Requirements section
• Add troubleshooting hint in the shell hook section for discoverability

Ubuntu and some other distributions default to ptrace_scope=1, which silently prevents pidfd_getfd from adopting PTY sessions — causing snag list to always return empty after hook setup.