fix: improve sed portability across GNU and BSD implementations

[kundeng]

Summary

• Fix sed compatibility issues that occur when users have GNU sed installed (common on macOS via Homebrew)
• Replace OS-based detection with portable temp file approach
• Eliminates "can't read s|pattern|replacement|: No such file or directory" errors

Problem

The original code used uname to detect macOS and assumed BSD sed, but:

• Homebrew users on macOS often install GNU sed as default
• GNU sed and BSD sed have different -i (in-place) syntax
• This caused sed to misinterpret replacement patterns as filenames

Solution

• Use temp file approach: sed pattern file > temp && mv temp file
• Works consistently across both GNU and BSD sed implementations
• No more OS-specific conditional logic needed
• Eliminates backup file syntax differences

Test plan

• Tested with GNU sed (Homebrew default)
• Tested with BSD sed (/usr/bin/sed)
• Verified harbor.sh ln works without errors
• Confirmed no backup files are created
• Rebased against latest main branch