feat: make cxcc installation opt-in

[Tim-1e]

Summary

• make cxcc installation opt-in while keeping the existing INSTALL_CXCC switch
• prompt interactive installs with a default-No choice and skip non-interactive installs by default
• pass INSTALL_CXCC=1 in CI paths that intentionally exercise a full cxcc install
• document the behavior in all maintained README translations

Validation

• PowerShell and Shell cxcc consumer smoke tests
• Shell pseudo-terminal default-No prompt smoke
• PowerShell syntax, profile smoke, and Windows full smoke
• Bash syntax and Termux platform smoke
• Gitleaks, public-repository hygiene, and git diff checks