feat(amazonq): Add conversation persistence to agentic chat

[tsmithsz]

Problem

Users lose all chats when they close VSCode, and there's no way to browse through chat history. Users also cant export their conversations to an easily shareable format.

Solution

Automatically persist conversations to JSON files in ~/.aws/amazonq/history, one for each workspace where Amazon Q chats occur. Add chat history and chat export buttons to top of Amazon Q toolbar. Clicking on the chat history button allows users to browse and search through chat history. Users click on an old conversation to open it back up (currently open conversations are in bold). Clicking on chat export button allows users to save chat transcript as a markdown or html.

Note: persistence + history is only for Q Chat Tabs (not /dev, /doc, /transform, etc.)

Note

Agentic chat does not currently use this persisted history in it's context. Follow-up PR will address this issue.

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[github-actions]

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