ci: block merges that drop coverage

Make the codecov project status a failing check (target auto, 0%
threshold) instead of informational, so any drop in include/ + src/
coverage fails it. Requires the check to be marked required in branch
protection. ci.yml already uploads coverage on every PR.

Author: sgerbino

.codecov.yml

@@ -12,12 +12,12 @@ codecov:
     after_n_builds: 1
     wait_for_ci: yes
 
-# Make coverage checks informational (report but never fail CI)
 coverage:
   status:
     project:
       default:
-        informational: true
+        target: auto
+        threshold: 0%
     patch:
       default:
         informational: true