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

docs/DOCKER.md

@@ -0,0 +1,75 @@
+# Docker
+
+DeepSeek TUI ships an official multi-arch Docker image (amd64 + arm64) on
+[GitHub Container Registry](https://github.com/Hmbown/DeepSeek-TUI/pkgs/container/deepseek-tui).
+
+## Quick start
+
+```bash
+docker run --rm -it \
+  -e DEEPSEEK_API_KEY="$DEEPSEEK_API_KEY" \
+  -v ~/.deepseek:/home/deepseek/.deepseek \
+  ghcr.io/hmbown/deepseek-tui:latest
+```
+
+Images are published to GitHub Container Registry (GHCR) only. Docker Hub
+publishing is not currently configured — add a `docker/login-action` step
+with Hub credentials to the release workflow if needed.
+
+## Environment variables
+
+| Variable              | Required | Description                                      |
+|-----------------------|----------|--------------------------------------------------|
+| `DEEPSEEK_API_KEY`    | yes      | DeepSeek API key                                 |
+| `DEEPSEEK_BASE_URL`   | no       | Custom API base URL (e.g. `https://api.deepseek.com`) |
+| `DEEPSEEK_NO_COLOR`   | no       | Set to `1` to disable terminal colour output     |
+
+## Volumes
+
+Mount `~/.deepseek` to persist sessions, config, skills, memory, and the offline queue
+across container restarts:
+
+```bash
+-v ~/.deepseek:/home/deepseek/.deepseek
+```
+
+Without this mount the container starts fresh each time.
+
+## Non-interactive / pipeline usage
+
+When stdin is not a TTY, `deepseek` drops to the dispatcher's one-shot mode
+(`deepseek -c "…"`). Pipe a prompt on stdin:
+
+```bash
+echo "Explain the Cargo.toml in structured English." | \
+  docker run --rm -i -e DEEPSEEK_API_KEY ghcr.io/hmbown/deepseek-tui:latest
+```
+
+## Building locally
+
+```bash
+# Single platform (your host architecture)
+docker build -t deepseek-tui .
+
+# Multi-platform (requires a builder with emulation)
+docker buildx create --use
+docker buildx build --platform linux/amd64,linux/arm64 -t deepseek-tui .
+```
+
+## Devcontainer
+
+The repository includes a [`.devcontainer/devcontainer.json`](../.devcontainer/devcontainer.json)
+configuration for VS Code / GitHub Codespaces. It pre-installs the Rust toolchain,
+rust-analyzer, and the `deepseek` binary. Open the repo in a devcontainer to get a
+ready-to-use development environment.
+
+## Tags
+
+| Tag        | Meaning                  |
+|------------|--------------------------|
+| `latest`   | Latest stable release    |
+| `v0`       | Latest v0.x release      |
+| `0.8.9`    | Specific release version |
+
+Docker images are built and pushed automatically when a release tag is pushed
+(see [release.yml](../.github/workflows/release.yml)).