fix(skills): drop spawn's forbidden empty related-skills field

Refuter finding 4 - naming-conventions forbids empty metadata fields;
omit rather than empty-string. (Missed in prior commit: sd pattern
failed silently on CRLF.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

skills/spawn/SKILL.md

@@ -6,7 +6,6 @@ compatibility: "Requires internet access for WebSearch/WebFetch to research offi
 allowed-tools: "Read Write Bash WebSearch WebFetch AskUserQuestion"
 metadata:
   author: claude-mods
-  related-skills: ""
 ---
 
 # Spawn - Expert Agent Generator