docs: drop the Quickstart block from all 25 pages

aihero.dev renders an install widget above every skill page. Each page
then repeated the same commands in its body, and the two copies had
drifted: the widget uses the current `npx skills@latest ...` wording
while the hand-written blocks mostly carried the older bare `npx skills
...`. Pages showed the right command and a stale one together.

Delete the block from all 25 pages and record the rule in
.agents/writing-docs.md: install wording belongs to the site, not the
page. The template no longer carries a Quickstart, and
.agents/install-block.md notes that docs pages are not one of its
consumers.

.agents/install-block.md

@@ -36,7 +36,7 @@ Pick the skills you want, and which coding agents to install them on. **The inst
 
 </canonical-block>
 
-…and the single-skill form on a `docs/` page, where the page already names one skill:
+…and the single-skill form wherever one skill is named on its own. Note that **`docs/` pages are not a consumer of this block**: ai-hero renders the install widget above the body, so a page that writes the commands out duplicates it. See [writing-docs.md](./writing-docs.md).
 
 <canonical-block name="skills-sh-one-skill">
 
@@ -50,7 +50,7 @@ npx skills@latest update <name>
 
 </canonical-block>
 
-`skills@latest` is the pinned spelling in all three. The Quickstart template in [writing-docs.md](./writing-docs.md), and the 24 pages under `docs/`, still carry the older bare `npx skills …`; the docs pass brings them into line.
+`skills@latest` is the pinned spelling in all three. The pages under `docs/` used to carry their own copy of these commands; those blocks are now deleted rather than corrected, because the site renders the install commands itself.
 
 ## The two routes are exclusive