feat(hooks): add agent-usage-reminder hook for background agent recommendations

Implements hook that tracks whether explore/librarian agents have been used in a session.
When target tools (Grep, Glob, WebFetch, context7, websearch_exa, grep_app) are called
without prior agent usage, appends reminder message recommending parallel background_task calls.

State persists across tool calls and resets on session compaction, allowing fresh reminders
after context compaction - similar to directory-readme-injector pattern.

Files:
- src/hooks/agent-usage-reminder/: New hook implementation
  - types.ts: AgentUsageState interface
  - constants.ts: TARGET_TOOLS, AGENT_TOOLS, REMINDER_MESSAGE
  - storage.ts: File-based state persistence with compaction handling
  - index.ts: Hook implementation with tool.execute.after and event handlers
- src/config/schema.ts: Add 'agent-usage-reminder' to HookNameSchema
- src/hooks/index.ts: Export createAgentUsageReminderHook
- src/index.ts: Instantiate and register hook handlers

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/index.ts

@@ -17,6 +17,7 @@ import {
   createBackgroundNotificationHook,
   createAutoUpdateCheckerHook,
   createUltraworkModeHook,
+  createAgentUsageReminderHook,
 } from "./hooks";
 import { createGoogleAntigravityAuthPlugin } from "./auth/antigravity";
 import {
@@ -207,6 +208,9 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
   const ultraworkMode = isHookEnabled("ultrawork-mode")
     ? createUltraworkModeHook()
     : null;
+  const agentUsageReminder = isHookEnabled("agent-usage-reminder")
+    ? createAgentUsageReminderHook(ctx)
+    : null;
 
   updateTerminalTitle({ sessionId: "main" });
 
@@ -320,6 +324,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await thinkMode?.event(input);
       await anthropicAutoCompact?.event(input);
       await ultraworkMode?.event(input);
+      await agentUsageReminder?.event(input);
 
       const { event } = input;
       const props = event.properties as Record<string, unknown> | undefined;
@@ -439,6 +444,7 @@ const OhMyOpenCodePlugin: Plugin = async (ctx) => {
       await directoryReadmeInjector?.["tool.execute.after"](input, output);
       await rulesInjector?.["tool.execute.after"](input, output);
       await emptyTaskResponseDetector?.["tool.execute.after"](input, output);
+      await agentUsageReminder?.["tool.execute.after"](input, output);
 
       if (input.sessionID === getMainSessionID()) {
         updateTerminalTitle({