Split HookContext and HookCommandContext to prevent deadlocks

HookContext (all events):
- isIdle() - read-only state check
- hasQueuedMessages() - read-only state check
- abort() - fire-and-forget, does not wait

HookCommandContext (slash commands only):
- waitForIdle() - waits for agent to finish
- newSession(options?) - create new session
- branch(entryId) - branch from entry
- navigateTree(targetId, options?) - navigate session tree

Session control methods moved from HookAPI (pi.*) to HookCommandContext (ctx.*)
because they can deadlock when called from event handlers that run inside
the agent loop (tool_call, tool_result, context events).

packages/coding-agent/src/core/hooks/index.ts

@@ -3,8 +3,11 @@ export {
 	discoverAndLoadHooks,
 	loadHooks,
 	type AppendEntryHandler,
+	type BranchHandler,
 	type LoadedHook,
 	type LoadHooksResult,
+	type NavigateTreeHandler,
+	type NewSessionHandler,
 	type SendMessageHandler,
 } from "./loader.js";
 export { execCommand, HookRunner, type HookErrorListener } from "./runner.js";