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.

.golangci.yml

@@ -0,0 +1,22 @@
+version: "2"
+run:
+  allow-parallel-runners: true
+linters:
+  exclusions:
+    generated: lax
+    presets:
+      - comments
+      - common-false-positives
+      - legacy
+      - std-error-handling
+    paths:
+      - third_party$
+      - builtin$
+      - examples$
+formatters:
+  exclusions:
+    generated: lax
+    paths:
+      - third_party$
+      - builtin$
+      - examples$