docs(skill): update canvas A2UI guidance

skills/clawdis-canvas/SKILL.md

@@ -1,6 +1,6 @@
 ---
 name: clawdis-canvas
-description: Drive the Clawdis Canvas panel (present, eval, snapshot, A2UI) via the clawdis CLI.
+description: Drive the Clawdis Canvas panel (present, eval, snapshot, A2UI) via the clawdis CLI, including gateway-hosted A2UI surfaces and action bridging.
 metadata: {"clawdis":{"always":true}}
 ---
 
@@ -21,3 +21,28 @@ A2UI
 Notes
 - Keep HTML under `~/clawd/canvas` when targeting remote nodes.
 - Use snapshot after renders to verify UI state.
+- Treat A2UI as gateway-hosted at `http(s)://<gateway-host>:<canvasPort>/__clawdis__/a2ui/`.
+- Rely on `canvas a2ui push/reset` to auto-navigate the Canvas to the gateway-hosted A2UI page.
+- Expect A2UI to fail if the Gateway does not advertise `canvasHostUrl` or is unreachable:
+  - `A2UI_HOST_NOT_CONFIGURED`
+  - `A2UI_HOST_UNAVAILABLE`
+
+A2UI quick flow
+1. Ensure the Gateway is running and reachable from the node.
+2. Build JSONL with **v0.8** server→client messages (`beginRendering`, `surfaceUpdate`, `dataModelUpdate`, `deleteSurface`).
+   - Do not use v0.9 `createSurface` (unsupported).
+3. Push JSONL and (optionally) snapshot the result.
+
+Example JSONL (v0.8)
+```bash
+cat > /tmp/a2ui-v0.8.jsonl <<'EOF'
+{"surfaceUpdate":{"surfaceId":"main","components":[{"id":"root","component":{"Column":{"children":{"explicitList":["title","content"]}}}},{"id":"title","component":{"Text":{"text":{"literalString":"A2UI (v0.8)"},"usageHint":"h1"}}},{"id":"content","component":{"Text":{"text":{"literalString":"If you can read this, A2UI is live."},"usageHint":"body"}}}]}}
+{"beginRendering":{"surfaceId":"main","root":"root"}}
+EOF
+
+clawdis canvas a2ui push --jsonl /tmp/a2ui-v0.8.jsonl --node <id>
+```
+
+Action callbacks (A2UI → agent)
+- A2UI user actions (buttons, etc.) are bridged from the WebView back to the node via `clawdisCanvasA2UIAction`.
+- Handle them on the agent side as `CANVAS_A2UI` messages (node → gateway → agent).