refine: Update terminology from Commands/Skills to User-invoked/Model-invoked and enhance documentation

skills/productivity/writing-great-skills/SKILL.md

@@ -33,6 +33,8 @@ Push too little down and the top bloats; push too much and you hide material the
 
 **Progressive disclosure** is the move down the ladder — out of `SKILL.md` into a linked file — so the top stays legible. Mechanics: a linked `.md` file in the skill folder, named for what it holds (this skill discloses its full definitions to `GLOSSARY.md`). Some skills are used in more than one way, and each distinct way is a **branch** — different runs taking different paths through the skill. Branching is the cleanest disclosure test: inline what every branch needs, and push behind a pointer what only some branches reach. A **context pointer**'s _wording_, not its target, decides when and how reliably the agent reaches the material.
 
+Where the ladder decides _how far down_ a piece sits, **co-location** decides _what sits beside it_ once there: keep a concept's definition, rules, and caveats under one heading rather than scattered, so reading one part brings its neighbours with it.
+
 ## When to split
 
 **Granularity** is how finely you divide skills, and each cut spends one of the two loads, so split only when the cut earns it. Two cuts: