Rename "design tree" → "decision tree" across skills and docs

Align the remaining callers and docs pages with grilling's body, which now
says "decision tree".

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/productivity/grill-me.md

@@ -22,7 +22,7 @@ You invoke this by typing `/grill-me` — the agent won't reach for it on its ow
 
 Reach for it before you build, when a plan feels roughly right but you can sense unresolved decisions hiding in it — the moment you want the soft spots found and forced into the open. If you want that same interrogation to also leave a paper trail of ADRs and a glossary behind, use [grill-with-docs](https://aihero.dev/skills-grill-with-docs) instead. And if the effort is too big to hold in one session and the route to the goal is still foggy — a greenfield project, a huge feature build — start further upstream with [wayfinder](https://aihero.dev/skills-wayfinder), which charts it as a map of decisions first and then merges back into this flow.
 
-## The design tree
+## The decision tree
 
 The session walks the plan as a tree of decisions, resolving dependencies between them one by one — a parent decision settled before the choices that hang off it. The point is not to reach agreement quickly; it is to make every implicit call explicit, so nothing important is left silently assumed. You come out the other side with a plan whose branches have all been visited.