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/config/schema.ts

@@ -41,6 +41,7 @@ export const HookNameSchema = z.enum([
   "background-notification",
   "auto-update-checker",
   "ultrawork-mode",
+  "agent-usage-reminder",
 ])
 
 export const AgentOverrideConfigSchema = z.object({