Revise README for Linux Privilege Escalation Lab

Refine mitigation and hardening recommendations

Add root cause analysis of SUID failure and capability abuse

Add SUID exploitation assessment and failure analysis

Add structured privilege escalation walkthrough

Document privilege escalation via cap_setuid capability abuse

Add capbash.c for privilege escalation exploit

Add vulnerable code example of SUID in vuln.c

Document why SUID PATH hijacking fails on Ubuntu 22.04

Add lab architecture and threat model documentation

Initial commit