chore(ci): add shared-lib scaffolding and functional coverage gate

Bring the auth and logging libs up to the otelsetup/authz_client standard:
golangci-lint + .editorconfig + .testcoverage.yml + cliff.toml + renovate.json
+ CHANGELOG + CLAUDE.md + pre-commit and Release workflows. Replace the
minimal test-only CI with a cache-based coverage-regression gate (PR test job
restores main's baseline from the Actions cache; a non-gating post-merge
coverage-baseline job records it) mirroring the services (ADR-0010 carve-out).
Job names test/vulnerabilities preserved to match branch-protection contexts.

.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$