Add OPEN_AI_CHAT postMessage event type to allow custom plugins to trigger the AI Assistant sidepane with a pre-populated prompt

Description:

Extend the Port plugin postMessage API with a new event type that allows a custom plugin to programmatically open the AI chat sidepane and optionally pre-populate it with a prompt.

Proposed message shape:

{

"type": "OPEN_AI_CHAT",

"data": {

"prompt": "Analyze the DORA metrics for this team and summarize change lead time trends.",

"agentIdentifier": "dora_metrics_agent"

}

}

prompt (string, optional): Pre-populated message to send or display in the chat input.

agentIdentifier (string, optional): If provided, opens the chat scoped to a specific AI agent. If omitted, opens the general Port AI assistant.