fix(install): allow interactive sudo from terminal installs

[debidong]

Summary

• allow install.sh to use normal interactive sudo when the installer is attached to a terminal
• keep the existing non-interactive fallback to $HOME/.local/bin for CI/agent/pipe-only contexts
• add installer behavior tests covering both non-TTY fallback and TTY interactive sudo paths
• run the installer behavior test from the install-scripts workflow

Test plan

• sh tests/install_sh_sudo_test.sh
• sh -n install.sh && bash -n install.sh && sh -n tests/install_sh_sudo_test.sh && bash -n tests/install_sh_sudo_test.sh
• go test -count=1 ./...
• pre-push hook: go test ./...

Notes

• The previous fallback-only behavior was introduced in v1.3.2 to avoid hanging non-interactive installs. This keeps that protection while restoring sudo prompts for real terminal installs.