feat: Bun single-file executable distribution (#819)

* feat: add Bun single-file executable distribution

- Add 7 platform packages for standalone CLI binaries
- Add bin/platform.js for shared platform detection
- Add bin/oh-my-opencode.js ESM wrapper
- Add postinstall.mjs for binary verification
- Add script/build-binaries.ts for cross-compilation
- Update publish workflow for multi-package publishing
- Add CI guard against @ast-grep/napi in CLI
- Add unit tests for platform detection (12 tests)
- Update README to remove Bun runtime requirement

Platforms supported:
- macOS ARM64 & x64
- Linux x64 & ARM64 (glibc)
- Linux x64 & ARM64 (musl/Alpine)
- Windows x64

Closes #816

* chore: remove unnecessary @ast-grep/napi CI check

* chore: gitignore compiled platform binaries

* fix: use require() instead of top-level await import() for Bun compile compatibility

* refactor: use static ESM import for package.json instead of require()

postinstall.mjs

@@ -0,0 +1,43 @@
+// postinstall.mjs
+// Runs after npm install to verify platform binary is available
+
+import { createRequire } from "node:module";
+import { getPlatformPackage, getBinaryPath } from "./bin/platform.js";
+
+const require = createRequire(import.meta.url);
+
+/**
+ * Detect libc family on Linux
+ */
+function getLibcFamily() {
+  if (process.platform !== "linux") {
+    return undefined;
+  }
+  
+  try {
+    const detectLibc = require("detect-libc");
+    return detectLibc.familySync();
+  } catch {
+    return null;
+  }
+}
+
+function main() {
+  const { platform, arch } = process;
+  const libcFamily = getLibcFamily();
+  
+  try {
+    const pkg = getPlatformPackage({ platform, arch, libcFamily });
+    const binPath = getBinaryPath(pkg, platform);
+    
+    // Try to resolve the binary
+    require.resolve(binPath);
+    console.log(`✓ oh-my-opencode binary installed for ${platform}-${arch}`);
+  } catch (error) {
+    console.warn(`⚠ oh-my-opencode: ${error.message}`);
+    console.warn(`  The CLI may not work on this platform.`);
+    // Don't fail installation - let user try anyway
+  }
+}
+
+main();