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

.devcontainer/devcontainer.json

@@ -0,0 +1,34 @@
+{
+  "name": "DeepSeek TUI",
+  "dockerFile": "../Dockerfile",
+  "build": {
+    "args": {
+      "RUST_VERSION": "1.85"
+    }
+  },
+  "customizations": {
+    "vscode": {
+      "extensions": [
+        "rust-lang.rust-analyzer",
+        "tamasfe.even-better-toml",
+        "vadimcn.vscode-lldb"
+      ],
+      "settings": {
+        "rust-analyzer.cargo.features": "all",
+        "editor.formatOnSave": true
+      }
+    }
+  },
+  "remoteEnv": {
+    "DEEPSEEK_API_KEY": "${localEnv:DEEPSEEK_API_KEY}"
+  },
+  "mounts": [
+    "source=${localEnv:HOME}/.deepseek,target=/home/deepseek/.deepseek,type=bind,consistency=cached"
+  ],
+  "features": {
+    "ghcr.io/devcontainers/features/rust:1": {},
+    "ghcr.io/devcontainers/features/git:1": {}
+  },
+  "postCreateCommand": "cargo build",
+  "remoteUser": "deepseek"
+}