feat(hooks): add auto-update-checker for plugin version management

Checks npm registry for latest version on session.created, invalidates
cache and shows toast notification when update is available.

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

src/hooks/auto-update-checker/index.ts

@@ -0,0 +1,56 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import { checkForUpdate } from "./checker"
+import { invalidateCache } from "./cache"
+import { PACKAGE_NAME } from "./constants"
+import { log } from "../../shared/logger"
+
+export function createAutoUpdateCheckerHook(ctx: PluginInput) {
+  let hasChecked = false
+
+  return {
+    event: async ({ event }: { event: { type: string; properties?: unknown } }) => {
+      if (event.type !== "session.created") return
+      if (hasChecked) return
+
+      const props = event.properties as { info?: { parentID?: string } } | undefined
+      if (props?.info?.parentID) return
+
+      hasChecked = true
+
+      try {
+        const result = await checkForUpdate(ctx.directory)
+
+        if (result.isLocalDev) {
+          log("[auto-update-checker] Skipped: local development mode")
+          return
+        }
+
+        if (!result.needsUpdate) {
+          log("[auto-update-checker] No update needed")
+          return
+        }
+
+        invalidateCache()
+
+        await ctx.client.tui
+          .showToast({
+            body: {
+              title: `${PACKAGE_NAME} Update`,
+              message: `v${result.latestVersion} available (current: v${result.currentVersion}). Restart OpenCode to apply.`,
+              variant: "info" as const,
+              duration: 8000,
+            },
+          })
+          .catch(() => {})
+
+        log(`[auto-update-checker] Update notification sent: v${result.currentVersion} → v${result.latestVersion}`)
+      } catch (err) {
+        log("[auto-update-checker] Error during update check:", err)
+      }
+    },
+  }
+}
+
+export type { UpdateCheckResult } from "./types"
+export { checkForUpdate } from "./checker"
+export { invalidateCache } from "./cache"