fix: update validate-issue handoff to use implement-task

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.codex/skills/validate-issue/SKILL.md

@@ -63,4 +63,4 @@ description: Phase 1 skill. Use when a Gitea issue is created or updated and nee
 - Do NOT proceed to Phase 2 — that is a separate skill.
 
 ## Next skill
-After Final Requirements are posted, the user invokes `/do-task <issue-id>` on the Developer agent.
+After Final Requirements are posted, the user invokes `/implement-task <issue-id>` on the Developer agent.