fix(auto-update): treat only explicit semver pins as user-pinned

Fixes #1920

Installer-written exact versions (e.g., oh-my-opencode@3.5.2) were incorrectly treated as user-pinned, blocking auto-updates for all installer users.

Fix isPinned to only block auto-update when pinnedVersion is an explicit semver string (user's intent). Channel tags (latest, beta, next) and bare package name all allow auto-update.

Fix installer fallback to return bare PACKAGE_NAME for stable versions and PACKAGE_NAME@{channel} for prerelease versions, preserving channel tracking.

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

@@ -7,6 +7,10 @@ import { extractChannel } from "../version-channel"
 import { findPluginEntry, getCachedVersion, getLatestVersion, revertPinnedVersion } from "../checker"
 import { showAutoUpdatedToast, showUpdateAvailableToast } from "./update-toasts"
 
+function getPinnedVersionToastMessage(latestVersion: string): string {
+  return `Update available: ${latestVersion} (version pinned, update manually)`
+}
+
 async function runBunInstallSafe(): Promise<boolean> {
   try {
     return await runBunInstall()
@@ -56,7 +60,7 @@ export async function runBackgroundUpdateCheck(
   }
 
   if (pluginInfo.isPinned) {
-    await showUpdateAvailableToast(ctx, latestVersion, getToastMessage)
+    await showUpdateAvailableToast(ctx, latestVersion, () => getPinnedVersionToastMessage(latestVersion))
     log(`[auto-update-checker] User-pinned version detected (${pluginInfo.entry}), skipping auto-update. Notification only.`)
     return
   }