Fix install.sh POSIX Shell Compatibility

[kaissb]

Branch: feature/0.1.1-fix-install-sh
Version: v0.1.1 — Gate Completion

Build

1. Change shebang to #!/usr/bin/env bash (pipefail requires bash).
2. Test on bash and dash.

Tests

• bash scripts/install.sh --help
• dash scripts/install.sh --help (if available)

Commit message

fix: install.sh POSIX shell compatibility

- Change shebang to #!/usr/bin/env bash
- Ensure pipefail works correctly
- Test on bash and dash