feat: implement user agents (top-level tabs via summon)

- agent parameter is now required on summon tool
- new top_level param spawns independent fire-and-forget user agent tabs
- gated by perm_user_agent permission (UI checkbox added)
- agent definition type validation (subagent vs user-agent slug mismatch)
- context-aware error messages when agent slug not found
- read_file_slice added to summon tool's allowed tools enum
- updated and expanded summon tests

1 files changed, 2 insertions, 1 deletions

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 9fe7550..3445301 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -79,7 +79,8 @@ export {
 	type AvailableAgent,
 	createSummonTool,
 	type SummonCallbacks,
-	toAvailableAgents,
+	toAvailableSubagents,
+	toAvailableUserAgents,
 } from "./tools/summon.js";
 export { createTaskListTool, TaskList } from "./tools/task-list.js";
 export { clearSpillForTab } from "./tools/truncate.js";