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/LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2025 Beads Contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.