fix(ci): docs-sync, electron linux fpm, docker cache env

CI Lint fixes:
- docs/openapi.yaml: bump version 2.2.0 → 2.2.3 (was out of sync with package.json)
- CHANGELOG.md: add '## [Unreleased]' as first section (required by check:docs-sync)

Electron Linux fix:
- electron-release.yml: add 'gem install fpm' step for Linux builds
  fpm is required by electron-builder to package .deb installers;
  ubuntu-latest runners don't have it pre-installed

Docker publish:
- docker-publish.yml: add DOCKER_BUILDKIT_INLINE_CACHE env; prev 502 was
  a transient Docker Hub network error, no code change needed

docs/openapi.yaml

@@ -1,7 +1,7 @@
 openapi: 3.1.0
 info:
   title: OmniRoute API
-  version: 2.2.0
+  version: 2.2.3
   description: |
     OmniRoute is a local-first AI API proxy router. It provides an OpenAI-compatible
     endpoint that routes requests to multiple AI providers with load balancing,