Add ToolRenderResult interface for custom tool rendering

- Changed ToolRenderer return type from TemplateResult to ToolRenderResult
- ToolRenderResult = { content: TemplateResult, isCustom: boolean }
- isCustom: true = no card wrapper, false = wrap in card
- Updated all existing tool renderers to return new format
- Updated Messages.ts to handle custom rendering

This enables tools to render without default card chrome when needed.

packages/ai/src/agent/index.ts

@@ -1,3 +1,3 @@
 export { agentLoop } from "./agent-loop.js";
 export * from "./tools/index.js";
-export type { AgentContext, AgentEvent, AgentTool, PromptConfig } from "./types.js";
+export type { AgentContext, AgentEvent, AgentLoopConfig, AgentTool, QueuedMessage } from "./types.js";