feat(snix/shell.nix): automatically set up the commit-msg hook

Lix does this and it has eliminated anyone ever having to think about
it; it just works.

Change-Id: I24a35d2d0b31f1245172d332bac40fe0b39dc2b9
Reviewed-on: https://cl.snix.dev/c/snix/+/30604
Tested-by: besadii
Reviewed-by: Florian Klink <flokli@flokli.de>

web/content/docs/guides/contributing.md

@@ -56,10 +56,18 @@ git remote set-url origin "https://$USER@cl.snix.dev/a/snix"
 replicates fast enough, then update to --push only -->
 
 ### Install the commit-msg hook
+
+Setting up the `commit-msg` Git hook is done for you automagically the first time you enter the shell.
+If it didn't work for you for some reason, see the manual steps below.
+
 Gerrit uses a `commit-msg` hook to add a `Change-Id: …` field to each commit
 message if not present already. This allows Gerrit to identify new revisions /
 updates of old commits, and track them as new revisions of the same "CL" [^2].
 
+<details>
+
+<summary>Manually installing the hook</summary>
+
 To install the commit-msg hook, run the following from the repo root:
 
 ```bash
@@ -68,6 +76,8 @@ curl -Lo .git/hooks/commit-msg https://cl.snix.dev/tools/hooks/commit-msg
 chmod +x .git/hooks/commit-msg
 ```
 
+</details>
+
 {{< callout context="tip" title="Did you know?" icon="outline/rocket" >}}
 Gerrit refuses receiving commits without these `Change-Id: …` fields.