Add docker mcp server init command with ChatGPT App UI

Introduces `docker mcp server init <directory>` command to scaffold complete
MCP server projects with ChatGPT App integration.

Command features:
- Creates project structure in specified directory
- --language=go flag (only Go supported currently)
- Validates empty directory and language support
- Customizes all files based on directory name

Generated project includes:
- main.go: MCP server with greet tool and UI resource
- ui.html: Interactive ChatGPT App UI with dropdown (Hi/Hey/Hello) and form
- Dockerfile: Multi-stage build with git for Go dependencies
- compose.yaml: Streaming gateway on port 8811, auth disabled (DOCKER_MCP_IN_CONTAINER=1)
- catalog.yaml: Server registry entry
- go.mod: Module with MCP SDK v1.0.0
- README.md: Complete documentation

ChatGPT App features:
- UI registered as resource at ui://greeter/widget.html with text/html+skybridge MIME type
- Tool metadata: openai/outputTemplate, openai/widgetAccessible, openai/toolInvocation/invoking
- Greet tool with name and greetingType parameters
- Returns structured data for UI consumption
- UI uses window.openai.callTool() API for real tool invocation
- Dropdown, text input, styled button, animated responses

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

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

Author: kgprs