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/hooks/agent-usage-reminder/storage.ts

@@ -0,0 +1,42 @@
+import {
+  existsSync,
+  mkdirSync,
+  readFileSync,
+  writeFileSync,
+  unlinkSync,
+} from "node:fs";
+import { join } from "node:path";
+import { AGENT_USAGE_REMINDER_STORAGE } from "./constants";
+import type { AgentUsageState } from "./types";
+
+function getStoragePath(sessionID: string): string {
+  return join(AGENT_USAGE_REMINDER_STORAGE, `${sessionID}.json`);
+}
+
+export function loadAgentUsageState(sessionID: string): AgentUsageState | null {
+  const filePath = getStoragePath(sessionID);
+  if (!existsSync(filePath)) return null;
+
+  try {
+    const content = readFileSync(filePath, "utf-8");
+    return JSON.parse(content) as AgentUsageState;
+  } catch {
+    return null;
+  }
+}
+
+export function saveAgentUsageState(state: AgentUsageState): void {
+  if (!existsSync(AGENT_USAGE_REMINDER_STORAGE)) {
+    mkdirSync(AGENT_USAGE_REMINDER_STORAGE, { recursive: true });
+  }
+
+  const filePath = getStoragePath(state.sessionID);
+  writeFileSync(filePath, JSON.stringify(state, null, 2));
+}
+
+export function clearAgentUsageState(sessionID: string): void {
+  const filePath = getStoragePath(sessionID);
+  if (existsSync(filePath)) {
+    unlinkSync(filePath);
+  }
+}