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.

.editorconfig

@@ -0,0 +1,11 @@
+root = true
+
+[*]
+end_of_line = lf
+insert_final_newline = true
+charset = utf-8
+trim_trailing_whitespace = true
+
+[*.go]
+indent_style = tab
+indent_size = 2