fix(#2849): resolve platform binaries using current package name

The installer wrapper and postinstall script still hardcoded the old
oh-my-opencode-* platform package family. When users installed the renamed
npm package oh-my-openagent (for example via npx oh-my-openagent install on
WSL2/Linux), the main package installed correctly but the wrapper looked for
nonexistent optional binaries like oh-my-opencode-linux-x64.

Fix:
- derive packageBaseName from package.json at runtime in wrapper/postinstall
- thread packageBaseName through platform package candidate resolution
- keep legacy oh-my-opencode default as fallback
- add regression test for renamed package family

postinstall.mjs

@@ -22,15 +22,26 @@ function getLibcFamily() {
   }
 }
 
+function getPackageBaseName() {
+  try {
+    const packageJson = JSON.parse(readFileSync(new URL("../package.json", import.meta.url), "utf8"));
+    return packageJson.name || "oh-my-opencode";
+  } catch {
+    return "oh-my-opencode";
+  }
+}
+
 function main() {
   const { platform, arch } = process;
   const libcFamily = getLibcFamily();
+  const packageBaseName = getPackageBaseName();
   
   try {
     const packageCandidates = getPlatformPackageCandidates({
       platform,
       arch,
       libcFamily,
+      packageBaseName,
     });
 
     const resolvedPackage = packageCandidates.find((pkg) => {