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.

docs/engineering/to-spec.md

@@ -1,13 +1,3 @@
-Quickstart:
-
-```bash
-npx skills add mattpocock/skills --skill=to-spec
-```
-
-```bash
-npx skills update to-spec
-```
-
 [Source](https://github.com/mattpocock/skills/tree/main/skills/engineering/to-spec)
 
 ## What it does