Add AI Agent Chat Window

[petertzy]

Provide a built-in AI Agent chat window for users to interact with the editor.

• Add a dockable AI Agent chat panel
• Allow AI Agent to receive context from the current document
• Support commands like 'format this section' or 'generate summary'
• Enable AI Agent to insert content or suggestions directly
• Save chat history per document
• Provide safety/validation checks for AI Agent actions