ci: add code coverage integration

Add go-test-coverage for coverage threshold enforcement. Coverage data
is uploaded as artifacts on main branch and compared against baseline
in PRs using shell script that gracefully handles first run without
baseline. PR comments show coverage percentage.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.testcoverage.yml

@@ -0,0 +1,13 @@
+# Coverage configuration for go-test-coverage
+# https://github.com/vladopajic/go-test-coverage
+
+profile: coverage.txt
+
+threshold:
+  file: 0
+  package: 0
+  total: 0
+
+exclude:
+  paths:
+    - _test\.go$