docs(subagent-driven-development): add main branch red flag to Never list

Add explicit warning against starting implementation on main/master
branch without first using a worktree for isolation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

skills/subagent-driven-development/SKILL.md

@@ -199,6 +199,7 @@ Done!
 ## Red Flags
 
 **Never:**
+- Start implementation on main/master branch (use worktree first)
 - Skip reviews (spec compliance OR code quality)
 - Proceed with unfixed issues
 - Dispatch multiple implementation subagents in parallel (conflicts)