Add npm package for bd with native binaries (bd-febc)

Implements @beads/bd npm package for easy installation in Node.js
environments, especially Claude Code for Web.

Features:
- Automatic platform-specific binary download during postinstall
- CLI wrapper that invokes native bd binary
- Full feature parity with standalone bd
- Works with SessionStart hooks for auto-installation

Package structure:
- bin/bd.js: Node.js CLI wrapper
- scripts/postinstall.js: Downloads correct binary from GitHub releases
- scripts/test.js: Verification tests
- Comprehensive documentation (6 guides)

Published to npm: https://www.npmjs.com/package/@beads/bd

Benefits vs WASM:
- Full SQLite support (no custom VFS)
- Better performance (native vs WASM)
- Simpler implementation and maintenance
- All commands work identically

Closes bd-febc, bd-be7a, bd-e2e6, bd-f282, bd-87a0, bd-b54c

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

npm-package/bin/bd.js

@@ -0,0 +1,54 @@
+#!/usr/bin/env node
+
+const { spawn } = require('child_process');
+const path = require('path');
+const os = require('os');
+const fs = require('fs');
+
+// Determine the platform-specific binary name
+function getBinaryPath() {
+  const platform = os.platform();
+  const arch = os.arch();
+
+  let binaryName = 'bd';
+  if (platform === 'win32') {
+    binaryName = 'bd.exe';
+  }
+
+  // Binary is stored in the package's bin directory
+  const binaryPath = path.join(__dirname, binaryName);
+
+  if (!fs.existsSync(binaryPath)) {
+    console.error(`Error: bd binary not found at ${binaryPath}`);
+    console.error('This may indicate that the postinstall script failed to download the binary.');
+    console.error(`Platform: ${platform}, Architecture: ${arch}`);
+    process.exit(1);
+  }
+
+  return binaryPath;
+}
+
+// Execute the native binary with all arguments passed through
+function main() {
+  const binaryPath = getBinaryPath();
+
+  // Spawn the native bd binary with all command-line arguments
+  const child = spawn(binaryPath, process.argv.slice(2), {
+    stdio: 'inherit',
+    env: process.env
+  });
+
+  child.on('error', (err) => {
+    console.error(`Error executing bd binary: ${err.message}`);
+    process.exit(1);
+  });
+
+  child.on('exit', (code, signal) => {
+    if (signal) {
+      process.exit(1);
+    }
+    process.exit(code || 0);
+  });
+}
+
+main();