feat(v0.8.9): address all issues labeled v0.8.9

#551 — sidebar filters prior-session agents (from_prior_session)
#552 — status messages prioritise ↑ affordance over /queue
#553 — oversized paste consolidation to @mention file (+uuid suffix)
#523 — release.yml: add if: guard so release job doesn't skip on dispatch
#526 — verify cost_status side-channel is fully wired (already in place)
#554 — mouse/trackpad scroll now sets user_scrolled_during_stream
#522 — set RELEASE_TAG_PAT secret for auto-tag → release trigger
#504 — session-context panel (SidebarFocus::Context, config toggle, default off)
#501 — multi-arch Dockerfile (+BUILDPLATFORM pin) + devcontainer + release CI
#484 — docs/RUNTIME_API.md rewritten against actual runtime_api.rs endpoints
#482 — close v0.8.8 planning tracker

Fixes from review:
- RUNTIME_API.md: corrected endpoints (/v1/...), port (7878), doctor JSON schema (flat)
- Dockerfile: added --platform=$BUILDPLATFORM for native multi-arch builds
- docs/DOCKER.md: removed Docker Hub references (GHCR only)
- sidebar.rs: dropped unused _theme variable
- settings.rs: context_panel default changed to false
- app.rs: paste filename now includes 8-char uuid suffix to avoid collision

.github/workflows/release.yml

@@ -115,8 +115,50 @@ jobs:
         with:
           name: ${{ matrix.artifact_name }}
           path: ${{ matrix.artifact_name }}
+  docker:
+    needs: build
+    if: ${{ !cancelled() && needs.build.result == 'success' }}
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      packages: write
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up QEMU
+        uses: docker/setup-qemu-action@v3
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+      - name: Log in to GitHub Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ghcr.io
+          username: ${{ github.repository_owner }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+      - name: Extract metadata
+        id: meta
+        uses: docker/metadata-action@v5
+        with:
+          images: |
+            ghcr.io/${{ github.repository }}
+          tags: |
+            type=semver,pattern={{version}}
+            type=semver,pattern={{major}}.{{minor}}
+            type=semver,pattern=v{{major}}
+            type=ref,event=tag
+      - name: Build and push
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          platforms: linux/amd64,linux/arm64
+          push: true
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
   release:
     needs: build
+    if: ${{ !cancelled() && needs.build.result == 'success' }}
     runs-on: ubuntu-latest
     permissions:
       contents: write