docs(readme): collapse the npm-install alternative into a details section (#844)

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -53,7 +53,8 @@ curl -fsSL https://raw.githubusercontent.com/colbymchenry/codegraph/main/install
 irm https://raw.githubusercontent.com/colbymchenry/codegraph/main/install.ps1 | iex
 ```
 
-Already have Node? Use npm instead (works on any version):
+<details>
+<summary><b>Already have Node? Use npm instead (works on any version)</b></summary>
 
 ```bash
 npm i -g @colbymchenry/codegraph
@@ -63,6 +64,8 @@ npm i -g @colbymchenry/codegraph
 
 <sub>**Upgrade any time** with `codegraph upgrade` — it detects how you installed (bundle, npm, or npx) and updates in place. Add `--check` to see if an update is available, or `codegraph upgrade <version>` to pin one.</sub>
 
+</details>
+
 ### 2. Wire up your agent(s)
 
 In a **new terminal**, run the installer to connect CodeGraph to the agents you use: