Add Docker HTTP transport and docs

[gabeosx]

Summary

• add src/server.ts SSE/HTTP transport that boots a fresh XeroMcpServer instance per session, guards /mcp behind an
  optional MCP_API_KEY, and exposes /healthz
• ship a multi-stage Dockerfile and .dockerignore, trimming runtime dependencies and wiring a health check for container
  deployments
• document the new transport, env vars, and verification checklist in README.md, and surface MCP_API_KEY in .env.example
• include examples/http-read-only-test.mjs to exercise read-only tools over HTTP and tighten XeroMcpServer/ToolFactory wiring to support repeated instantiation
• trim credential environment variables in xero-client to avoid whitespace issues reported by Xero

Configuration changes

• introduce MCP_API_KEY (recommended for HTTP transport authentication)
• support optional HOST/PORT overrides when running dist/server.js or the Docker image