fix(auto-update): align PACKAGE_NAME with published package name (#3129)

Changed PACKAGE_NAME from 'oh-my-openagent' to 'oh-my-opencode' in
auto-update-checker/constants.ts to match package.json. This fixes
getCachedVersion() returning null, causing 'unknown' in version toast.

TDD: red-green confirmed. 39 tests pass, tsc clean.

Closes #3129

src/hooks/auto-update-checker/checker/plugin-entry.test.ts

@@ -3,8 +3,7 @@ import { spawnSync } from "node:child_process"
 import * as fs from "node:fs"
 import * as os from "node:os"
 import * as path from "node:path"
-
-const PACKAGE_NAME = "oh-my-openagent"
+import { PACKAGE_NAME } from "../constants"
 
 type PluginEntryResult = {
   entry: string