chore: import upstream snapshot with attribution

showcase/integrations/claude-sdk-python/qa/headless-simple.md

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+# QA: Headless Chat (Simple) — Claude Agent SDK (Python)
+
+## Prerequisites
+
+- Demo is deployed and accessible
+- Agent backend is healthy (check /api/health)
+
+## Test Steps
+
+### 1. Basic Functionality
+
+- [ ] Navigate to `/demos/headless-simple`
+- [ ] Verify the heading "Headless Chat (Simple)" is visible
+- [ ] Verify the placeholder text "No messages yet. Say hi!" is displayed
+- [ ] Type a message into the textarea
+- [ ] Click Send (or press Enter)
+- [ ] Verify the user bubble appears immediately on the right
+- [ ] Verify the agent responds with a text bubble on the left
+
+### 2. Feature-Specific Checks
+
+#### useAgent / useComponent
+
+- [ ] Ask "Show me a card about cats"
+- [ ] Verify the ShowCard component renders with a title and body
+
+#### Running state
+
+- [ ] While the agent is replying, verify "Agent is thinking..." appears below the message list
+- [ ] Verify the Send button is disabled while the agent is running
+
+### 3. Error Handling
+
+- [ ] Verify sending an empty message does nothing
+- [ ] Verify no console errors during normal usage
+
+## Expected Results
+
+- Page loads within 3 seconds
+- Agent responds within 10 seconds
+- ShowCard renders when the agent calls `show_card`