docs: update all markdown to v0.28.1 state

- README: add GHCR pre-built images to Docker section, update line counts
  and test count (426 tests, 22 files), add CI/CD to architecture tree
- ROADMAP: update header to v0.28.1/426 tests, mark all user-requested
  features as shipped, collapse completed Waves 3-7 into summary table,
  update architecture line counts, add CI/CD row
- CHANGELOG: add v0.28.1 entry for CI pipeline + multi-arch Docker builds,
  update footer version
- SPRINTS: update header and footer to v0.28.1

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,19 @@
 
 ---
 
+## [v0.28.1] CI Pipeline + Multi-Arch Docker Builds
+*April 3, 2026 | 426 tests*
+
+### Features
+- **GitHub Actions CI.** New workflow triggers on tag push (`v*`). Builds
+  multi-arch Docker images (linux/amd64 + linux/arm64), pushes to
+  `ghcr.io/nesquena/hermes-webui`, and creates a GitHub Release with
+  auto-generated release notes. Uses GHA layer caching for fast rebuilds.
+- **Pre-built container images.** Users can now `docker pull ghcr.io/nesquena/hermes-webui:latest`
+  instead of building locally.
+
+---
+
 ## [v0.27] Profile Creation Fallback for Docker (Issue #44)
 *April 3, 2026 | 426 tests*
 
@@ -904,4 +917,4 @@ Three-panel layout: sessions sidebar, chat area, workspace panel.
 
 ---
 
-*Last updated: v0.27, April 3, 2026 | Tests: 426*
+*Last updated: v0.28.1, April 3, 2026 | Tests: 426*