Improve shell script robustness and error handling

• Added set -euo pipefail to ensure the script exits on errors, undefined variables, or failed pipelines.
• Enhanced error handling for SSH agent start and key addition, with clear error messages.
• Replaced echo with printf for more reliable and formatted output.
• Introduced optional legacy RSA hostkey handling based on input parameters.
• Improved overall script readability with clearer variable names and structured conditionals.

Thank you for your contribution! Will review it

@Burnett01 any news?

looks good so far, I will get back to you