fix: use export type for type-only re-exports

Fixes build error with isolatedModules.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/agents/index.ts

@@ -17,4 +17,4 @@ export function getAgentSpec(kind: AgentKind): AgentSpec {
   return specs[kind];
 }
 
-export { AgentKind, AgentMeta, AgentParseResult } from "./types.js";
+export type { AgentKind, AgentMeta, AgentParseResult } from "./types.js";