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/scripts/test.js

@@ -0,0 +1,29 @@
+#!/usr/bin/env node
+
+const { execSync } = require('child_process');
+const path = require('path');
+
+function runTests() {
+  console.log('Testing bd installation...\n');
+
+  const bdPath = path.join(__dirname, '..', 'bin', 'bd.js');
+
+  try {
+    // Test 1: Version check
+    console.log('Test 1: Checking bd version...');
+    const version = execSync(`node "${bdPath}" version`, { encoding: 'utf8' });
+    console.log(`✓ Version check passed: ${version.trim()}\n`);
+
+    // Test 2: Help command
+    console.log('Test 2: Checking bd help...');
+    execSync(`node "${bdPath}" --help`, { stdio: 'pipe' });
+    console.log('✓ Help command passed\n');
+
+    console.log('✓ All tests passed!');
+  } catch (err) {
+    console.error('✗ Tests failed:', err.message);
+    process.exit(1);
+  }
+}
+
+runTests();