Let Dex Copilot triage connected email and surface replies due

Dex Copilot should be able to inspect messages from a connected email account and identify conversations that likely need a reply.

Current gap: Copilot cannot review an inbox or prioritize unanswered threads, so users cannot ask which email conversations need attention now.

Desired outcome: return a bounded, reviewable list of likely replies due, with enough thread context and a concise reason for each suggestion. Keep triage read-only by default and never send a reply without an explicit separate user action.