fix(auto-update): sync cache package.json to opencode.json intent

When users switch from pinned version to tag in opencode.json (e.g.,
3.10.0 -> @latest), the cache package.json still contains the resolved
version. This causes bun install to reinstall the old version instead
of resolving the new tag.

This adds syncCachePackageJsonToIntent() which updates the cache
package.json to match user intent before running bun install. Uses
atomic writes (temp file + rename) with UUID-based temp names for
concurrent safety.

Critical changes:
- Treat all sync errors as abort conditions (file_not_found,
  plugin_not_in_deps, parse_error, write_error) to prevent corrupting
  a bad cache state further
- Remove dead code (unreachable revert branch for pinned versions)
- Add tests for all error paths and atomic write cleanup

src/hooks/auto-update-checker/hook/background-update-check.ts

@@ -4,7 +4,7 @@ import { log } from "../../../shared/logger"
 import { invalidatePackage } from "../cache"
 import { PACKAGE_NAME } from "../constants"
 import { extractChannel } from "../version-channel"
-import { findPluginEntry, getCachedVersion, getLatestVersion, revertPinnedVersion } from "../checker"
+import { findPluginEntry, getCachedVersion, getLatestVersion, syncCachePackageJsonToIntent } from "../checker"
 import { showAutoUpdatedToast, showUpdateAvailableToast } from "./update-toasts"
 
 function getPinnedVersionToastMessage(latestVersion: string): string {
@@ -65,6 +65,17 @@ export async function runBackgroundUpdateCheck(
     return
   }
 
+  // Sync cache package.json to match opencode.json intent before updating
+  // This handles the case where user switched from pinned version to tag (e.g., 3.10.0 -> @latest)
+  const syncResult = syncCachePackageJsonToIntent(pluginInfo)
+
+  // Abort on ANY sync error to prevent corrupting a bad state further
+  if (syncResult.error) {
+    log(`[auto-update-checker] Sync failed with error: ${syncResult.error}`, syncResult.message)
+    await showUpdateAvailableToast(ctx, latestVersion, getToastMessage)
+    return
+  }
+
   invalidatePackage(PACKAGE_NAME)
 
   const installSuccess = await runBunInstallSafe()
@@ -75,11 +86,6 @@ export async function runBackgroundUpdateCheck(
     return
   }
 
-  if (pluginInfo.isPinned) {
-    revertPinnedVersion(pluginInfo.configPath, latestVersion, pluginInfo.entry)
-    log("[auto-update-checker] Config reverted due to install failure")
-  }
-
   await showUpdateAvailableToast(ctx, latestVersion, getToastMessage)
   log("[auto-update-checker] bun install failed; update not installed (falling back to notification-only)")
 }