Add Desktop Extensions (DXT) support with manifest and build workflow

[pppp606]

Summary

This PR adds support for Anthropic's Desktop Extensions (DXT) format, enabling one-click
installation of the Backlog MCP Server in Claude Desktop.

https://www.anthropic.com/engineering/desktop-extensions

Changes

• manifest.json: Complete DXT manifest with server configuration, user config, and tool
  definitions
• package.json: Add @anthropic-ai/dxt dev dependency for extension development
• Documentation: Updated README.md and README.ja.md with DXT installation options and
  development commands

Testing

# Validate manifest
npx dxt validate manifest.json

# Create DXT package
npx dxt pack

Code Signing Proposal

Currently, the generated DXT file is unsigned, which displays a security warning in Claude
Desktop. For production releases, we should implement proper code signing:

Immediate Options

1. Self-signed certificates for development/testing
2. Official signing by Nulab for production releases

We recommend adding a release workflow that automatically creates signed DXT files:

- name: Build and Sign DXT
  run: |
    npm run build
    npx dxt pack
    npx dxt sign backlog-mcp-server@${{ github.event.release.tag_name }}.dxt \
      --cert ${{ secrets.DXT_CERT }} \
      --key ${{ secrets.DXT_KEY }}

- name: Upload DXT to Release
  uses: actions/upload-release-asset@v1
  with:
    asset_path: ./backlog-mcp-server@${{ github.event.release.tag_name }}.dxt
    asset_name: backlog-mcp-server-${{ github.event.release.tag_name }}.dxt
    asset_content_type: application/zip