Drop-in Flutter widgets for AI agent UIs — tool-call cards, thinking
bubbles, citation chips, plan-progress timelines. Backend-agnostic.

- Core models: ToolCall, ThinkingStep, Citation, PlanStep + sealed AgentEvent
- Widgets: ToolCallCard, ToolCallFeed, ThinkingBubble, ReasoningTrace,
  CitationChip, CitationCard, CitationList, PlanProgress, AgentTimeline
- Composition: AgentView, AgentEventBuilder
- Theme: AgentKitTheme with light/dark + density presets
- 55 tests passing, 0 analyzer issues, dart pub publish dry-run clean