fix(auto-update-checker): use OpenCode cache paths for updates

Align version lookup, invalidation, and bun install with OpenCode's cache directory so updates target the loaded plugin location. Keep dependency declarations intact during invalidation so auto-update can reinstall instead of converging to uninstall.

src/hooks/auto-update-checker/constants.test.ts

@@ -0,0 +1,14 @@
+import { describe, expect, it } from "bun:test"
+import { join } from "node:path"
+import { getOpenCodeCacheDir } from "../../shared/data-path"
+
+describe("auto-update-checker constants", () => {
+  it("uses the OpenCode cache directory for installed package metadata", async () => {
+    const { CACHE_DIR, INSTALLED_PACKAGE_JSON, PACKAGE_NAME } = await import(`./constants?test=${Date.now()}`)
+
+    expect(CACHE_DIR).toBe(getOpenCodeCacheDir())
+    expect(INSTALLED_PACKAGE_JSON).toBe(
+      join(getOpenCodeCacheDir(), "node_modules", PACKAGE_NAME, "package.json")
+    )
+  })
+})