Add bash completion support and improve installation instructions

[mamahoos]

Summary

This PR adds bash completion support and improves the installation script for better reliability and portability.

Changes

• Add bash completion script for irdocker
• Improve install script structure and execution flow
• Fix incorrect self-install behavior (install ... same file)
• Use temporary build artifact instead of writing directly to system paths

Notes

• Completion files are installed into standard bash-completion directories when available
• No behavior change in runtime CLI logic