fix(cursor): update skill paths from .cursor/rules/ to .cursor/skills/

The Cursor Agent CLI now discovers skills from .cursor/skills/ instead
of the old .cursor/rules/ directory. Update all references across
create-plan and implement-plan Cursor variants and docs.

skills/create-plan/cursor/templates/milestone-plan.md

@@ -7,7 +7,7 @@
 
 ## Planning Guardrails
 - This plan assumes design was validated before implementation planning.
-- Skills are invoked via workspace discovery (`.cursor/rules/`).
+- Skills are invoked via workspace discovery (`.cursor/skills/`).
 - Milestones require verification + local commits + explicit approval before proceeding.
 
 ## Context