Update CopilotKit to 1.5.1

[jwoyo]

Migrated to the new CopilotKit hook APIs:

• useCoAgent → useAgent (v2 API)
• useCopilotAction → useHumanInTheLoop for chart actions
• useCopilotAction → useRenderToolCall for search rendering
• renderAndWaitForResponse → render

Also:

• bumped Next.js to latest (was still exploitable!)
• add type checking to lint script

UI component v2 updates might be coming in a follow-up PR if desired.

[vercel]

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