refactor: improve agent notification types and fix activity curator
- Convert agent notifications to discriminated union (session/permission/status)
- Fix activity curator to properly handle typed session updates
- Serialize Date objects in AgentInfo for JSON compatibility
- Use activity curator for title generation context
- Reduce console.error to console.warn for non-critical WebSocket errors
- Improve spacing in agent stream views
- Delay title generation until 15+ updates available
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