feat: add linux-musl (Alpine) builds for x64 and arm64 (#784)

* feat: add linux-musl (Alpine) builds for x64 and arm64

Add x86_64-unknown-linux-musl and aarch64-unknown-linux-musl targets to
the release workflow using cargo-zigbuild. Update the JS wrapper and
postinstall script to detect musl libc and select the correct binary.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: simplify isMusl() and add musl targets to build script

Address PR #784 review feedback:
- Remove redundant first try block in isMusl() (ldd --version always
  throws on musl, so only the `|| true` variant works)
- Add x86_64-unknown-linux-musl and aarch64-unknown-linux-musl targets
  to build-all-platforms.sh to stay in sync with the release workflow

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

bin/agent-browser.js

@@ -8,7 +8,7 @@
  * binary directly (zero overhead).
  */
 
-import { spawn } from 'child_process';
+import { spawn, execSync } from 'child_process';
 import { existsSync, accessSync, chmodSync, constants } from 'fs';
 import { dirname, join } from 'path';
 import { fileURLToPath } from 'url';
@@ -16,6 +16,17 @@ import { platform, arch } from 'os';
 
 const __dirname = dirname(fileURLToPath(import.meta.url));
 
+// Detect if the system uses musl libc (e.g. Alpine Linux)
+function isMusl() {
+  if (platform() !== 'linux') return false;
+  try {
+    const result = execSync('ldd --version 2>&1 || true', { encoding: 'utf8' });
+    return result.toLowerCase().includes('musl');
+  } catch {
+    return existsSync('/lib/ld-musl-x86_64.so.1') || existsSync('/lib/ld-musl-aarch64.so.1');
+  }
+}
+
 // Map Node.js platform/arch to binary naming convention
 function getBinaryName() {
   const os = platform();
@@ -27,7 +38,7 @@ function getBinaryName() {
       osKey = 'darwin';
       break;
     case 'linux':
-      osKey = 'linux';
+      osKey = isMusl() ? 'linux-musl' : 'linux';
       break;
     case 'win32':
       osKey = 'win32';