fix(observability): guard against nil tag before sort

site.tags["observability"] returns nil when no posts carry the tag,
causing Liquid's sort filter to throw 'Cannot sort a null object.' and
breaking the pre-commit Jekyll build. Guard with a nil check and fall
back to an empty array.

Broke the weekly gptme digest publish step (bob-weekly-gptme-digest.service).

Git-Session-Id: 0e108269-aed7-406a-9942-c991b20ebdb0

Author: TimeToBuildBob

observability.pug

@@ -62,7 +62,8 @@ main.container.mx-auto.px-4.py-8
         | {% endfor %}
 
     //- All observability-tagged posts by date
-    | {% assign obs_posts = site.tags["observability"] | sort: "date" | reverse %}
+    | {% assign obs_posts_raw = site.tags["observability"] %}
+    | {% if obs_posts_raw %}{% assign obs_posts = obs_posts_raw | sort: "date" | reverse %}{% else %}{% assign obs_posts = "" | split: "" %}{% endif %}
     .mb-4
       h2.text-lg.font-bold.mb-4.pb-2.border-b.border-border(class="text-text")
         | All posts