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/index.ts

@@ -0,0 +1,114 @@
+import type { PluginInput } from "@opencode-ai/plugin";
+import {
+  loadAgentUsageState,
+  saveAgentUsageState,
+  clearAgentUsageState,
+} from "./storage";
+import { TARGET_TOOLS, AGENT_TOOLS, REMINDER_MESSAGE } from "./constants";
+import type { AgentUsageState } from "./types";
+
+interface ToolExecuteInput {
+  tool: string;
+  sessionID: string;
+  callID: string;
+  parentSessionID?: string;
+}
+
+interface ToolExecuteOutput {
+  title: string;
+  output: string;
+  metadata: unknown;
+}
+
+interface EventInput {
+  event: {
+    type: string;
+    properties?: unknown;
+  };
+}
+
+export function createAgentUsageReminderHook(_ctx: PluginInput) {
+  const sessionStates = new Map<string, AgentUsageState>();
+
+  function getOrCreateState(sessionID: string): AgentUsageState {
+    if (!sessionStates.has(sessionID)) {
+      const persisted = loadAgentUsageState(sessionID);
+      const state: AgentUsageState = persisted ?? {
+        sessionID,
+        agentUsed: false,
+        reminderCount: 0,
+        updatedAt: Date.now(),
+      };
+      sessionStates.set(sessionID, state);
+    }
+    return sessionStates.get(sessionID)!;
+  }
+
+  function markAgentUsed(sessionID: string): void {
+    const state = getOrCreateState(sessionID);
+    state.agentUsed = true;
+    state.updatedAt = Date.now();
+    saveAgentUsageState(state);
+  }
+
+  function resetState(sessionID: string): void {
+    sessionStates.delete(sessionID);
+    clearAgentUsageState(sessionID);
+  }
+
+  const toolExecuteAfter = async (
+    input: ToolExecuteInput,
+    output: ToolExecuteOutput,
+  ) => {
+    const { tool, sessionID, parentSessionID } = input;
+
+    // Only run in root sessions (no parent = main session)
+    if (parentSessionID) {
+      return;
+    }
+
+    if ((AGENT_TOOLS as readonly string[]).includes(tool)) {
+      markAgentUsed(sessionID);
+      return;
+    }
+
+    if (!(TARGET_TOOLS as readonly string[]).includes(tool)) {
+      return;
+    }
+
+    const state = getOrCreateState(sessionID);
+
+    if (state.agentUsed) {
+      return;
+    }
+
+    output.output += REMINDER_MESSAGE;
+    state.reminderCount++;
+    state.updatedAt = Date.now();
+    saveAgentUsageState(state);
+  };
+
+  const eventHandler = async ({ event }: EventInput) => {
+    const props = event.properties as Record<string, unknown> | undefined;
+
+    if (event.type === "session.deleted") {
+      const sessionInfo = props?.info as { id?: string } | undefined;
+      if (sessionInfo?.id) {
+        resetState(sessionInfo.id);
+      }
+    }
+
+    if (event.type === "session.compacted") {
+      const sessionID = (props?.sessionID ??
+        (props?.info as { id?: string } | undefined)?.id) as string | undefined;
+      if (sessionID) {
+        resetState(sessionID);
+      }
+    }
+  };
+
+  return {
+    "tool.execute.after": toolExecuteAfter,
+    event: eventHandler,
+  };
+}