Reconfigure codecov

Bonus: this one is actually valid.

codecov.yml

@@ -1,9 +1,24 @@
+# Goals of this configuration:
+# * Support codecov informing us about where we need to improve
+# * Drive overall code coverage up over the long term
+# * Don't discourage contributors with Red X of Shame
+
 coverage:
   status:
     project:
-      enabled: false
+      # We want the project to be at least as well covered as
+      # it was before, but we'll allow a 1% drop without incident,
+      # because stuff happens.  It's on us to make sure we're
+      # improving over the long haul.
+      default:
+        target: auto
+        threshold: 1%
     patch:
-      enabled: false
-    changes:
-      enabled: false
-comment: off
+      # Patch coverage is informative, so we leave it enabled.
+      # But there are a lot of reasonable commits that don't affect
+      # overall coverage while have dreadful patch metrics (e.g.,
+      # reformatting).  Let's calculate this, but not punish it.
+      # If we want more tests, a human should politely ask for them.
+      default:
+        target: 0%
+    changes: no