Update web-ui CHANGELOG.md and README.md for v0.31.0

Breaking changes:
- Agent class moved to @mariozechner/pi-agent-core
- Transport abstraction removed (ProviderTransport, AppTransport)
- AppMessage renamed to AgentMessage
- UserMessageWithAttachments now has role: 'user-with-attachments'
- CustomMessages interface replaced with CustomAgentMessages
- agent.appendMessage() removed (use queueMessage())
- New agent event types from pi-agent-core

Added:
- defaultConvertToLlm, convertAttachments utilities
- isUserMessageWithAttachments, isArtifactMessage type guards
- createStreamFn for CORS proxy support
- Default streamFn and getApiKey on AgentInterface
- Proxy utilities exported

README completely rewritten to reflect new architecture.

packages/web-ui/CHANGELOG.md

@@ -14,6 +14,10 @@
 
 - **`CustomMessages` interface removed**: Use declaration merging on `CustomAgentMessages` from `@mariozechner/pi-agent-core` instead.
 
+- **`agent.appendMessage()` removed**: Use `agent.queueMessage()` instead.
+
+- **Agent event types changed**: `AgentInterface` now handles new event types from `@mariozechner/pi-agent-core`: `message_start`, `message_end`, `message_update`, `turn_start`, `turn_end`, `agent_start`, `agent_end`.
+
 ### Added
 
 - **`defaultConvertToLlm`**: Default message transformer that handles `UserMessageWithAttachments` and `ArtifactMessage`. Apps can extend this for custom message types.
@@ -38,6 +42,7 @@
 - `AgentTransport` interface
 - `AgentRunConfig` type
 - `ProxyAssistantMessageEvent` type
+- `test-sessions.ts` example file
 
 ### Migration Guide