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/cli/config-manager/plugin-name-with-version.ts

@@ -3,6 +3,15 @@ import { fetchNpmDistTags } from "./npm-dist-tags"
 const PACKAGE_NAME = "oh-my-opencode"
 const PRIORITIZED_TAGS = ["latest", "beta", "next"] as const
 
+function getFallbackEntry(version: string): string {
+  const prereleaseMatch = version.match(/-([a-zA-Z][a-zA-Z0-9]*)\./)
+  if (prereleaseMatch) {
+    return `${PACKAGE_NAME}@${prereleaseMatch[1]}`
+  }
+
+  return PACKAGE_NAME
+}
+
 export async function getPluginNameWithVersion(currentVersion: string): Promise<string> {
   const distTags = await fetchNpmDistTags(PACKAGE_NAME)
 
@@ -15,5 +24,5 @@ export async function getPluginNameWithVersion(currentVersion: string): Promise<
     }
   }
 
-  return `${PACKAGE_NAME}@${currentVersion}`
+  return getFallbackEntry(currentVersion)
 }