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>

.gitignore

@@ -57,3 +57,13 @@ dist/
 # Git worktrees
 .worktrees/
 .beads/pollution-backup.jsonl
+
+# npm package - exclude downloaded binaries and archives
+npm-package/bin/bd
+npm-package/bin/*.tar.gz
+npm-package/bin/*.zip
+npm-package/bin/CHANGELOG.md
+npm-package/bin/LICENSE
+npm-package/bin/README.md
+npm-package/node_modules/
+npm-package/package-lock.json