Fix uninstall.sh crash when executed via curl | sudo bash

[Copilot]

When piped into bash, uninstall.sh failed with a syntax error around line 79 because interactive read calls were competing with the script stream on stdin.

Changes

• Dedicated TTY descriptor — open /dev/tty once into a named FD at startup; fall back to empty string on failure:

  TTY_FD=""
  exec {TTY_FD}</dev/tty 2>/dev/null || TTY_FD=""

• Isolated prompt reads — ask_yes_no reads from <&$TTY_FD instead of stdin, eliminating stream contention
• Numeric FD guard — validate $TTY_FD =~ ^[0-9]+$ before any redirection or close to prevent unexpected expansion
• Explicit cleanup — close the descriptor before exit