Add Open VSIX build script

Build script that creates Open VSIX variant with different branding:
- Changes displayName to "Claude Code Chat"
- Uses icon.png instead of icon-bubble.png
- Automatically backs up and restores files after build

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.vscodeignore

@@ -13,4 +13,5 @@ backup
 .claude
 claude-code-chat-permissions-mcp/**
 node_modules
-mcp-permissions.js
+mcp-permissions.js
+build