Fix macOS BSD sed installer failure

[steffenms]

Summary

• Make installer sed usage portable across GNU and BSD/macOS sed.
• Replace fragile sed-based frontmatter injection with awk/temp-file rewriting while preserving destination file permissions.
• Keep generated delegate and implementer variant title updates compatible with BSD sed.

Test plan

• bash -n install.sh
• Reviewed changed installer paths for macOS/Linux sed compatibility.