ci: add path filters to Docker and test workflows, remove supply chain audit

- Docker build only triggers on main push (code/config changes) and
  releases, no longer on every PR
- Tests skip markdown-only and docs-only changes
- Remove supply-chain-audit workflow

.github/workflows/tests.yml

@@ -3,8 +3,14 @@ name: Tests
 on:
   push:
     branches: [main]
+    paths-ignore:
+      - '**/*.md'
+      - 'docs/**'
   pull_request:
     branches: [main]
+    paths-ignore:
+      - '**/*.md'
+      - 'docs/**'
 
 permissions:
   contents: read