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/constants.ts

@@ -0,0 +1,52 @@
+import { join } from "node:path";
+import { xdgData } from "xdg-basedir";
+
+export const OPENCODE_STORAGE = join(xdgData ?? "", "opencode", "storage");
+export const AGENT_USAGE_REMINDER_STORAGE = join(
+  OPENCODE_STORAGE,
+  "agent-usage-reminder",
+);
+
+export const TARGET_TOOLS = [
+  "Grep",
+  "safe_grep",
+  "Glob",
+  "safe_glob",
+  "WebFetch",
+  "context7_resolve-library-id",
+  "context7_get-library-docs",
+  "websearch_exa_web_search_exa",
+  "grep_app_searchGitHub",
+] as const;
+
+export const AGENT_TOOLS = [
+  "Task",
+  "call_omo_agent",
+  "background_task",
+] as const;
+
+export const REMINDER_MESSAGE = `
+[Agent Usage Reminder]
+
+You called a search/fetch tool directly without leveraging specialized agents.
+
+RECOMMENDED: Use background_task with explore/librarian agents for better results:
+
+\`\`\`
+// Parallel exploration - fire multiple agents simultaneously
+background_task(agent="explore", prompt="Find all files matching pattern X")
+background_task(agent="explore", prompt="Search for implementation of Y") 
+background_task(agent="librarian", prompt="Lookup documentation for Z")
+
+// Then continue your work while they run in background
+// System will notify you when each completes
+\`\`\`
+
+WHY:
+- Agents can perform deeper, more thorough searches
+- Background tasks run in parallel, saving time
+- Specialized agents have domain expertise
+- Reduces context window usage in main session
+
+ALWAYS prefer: Multiple parallel background_task calls > Direct tool calls
+`;