Add XBridge demo and feature tour

[chaiwithcode]

Summary

• Add a buildable SwiftUI demo project with a shared Xcode scheme, XCTest, and Swift Testing coverage.
• Add real VS Code and Simulator screenshots plus a compact three-scene GIF to the README.
• Document all 34 XBridge commands and eight AI agent tools.
• Keep the sample project's build products and local settings out of Git and exclude the demo project from the VSIX package.

Verification

• npm run package passed.
• xcodebuild test passed on the iPhone 17 Pro Max simulator.
• XBridge Build & Run launched the demo app in Simulator.
• Both demo tests passed in VS Code Test Explorer.
• git diff --check passed.