docs: expand workflow guide with tools, variable refs, and extension patterns

Added comprehensive documentation covering:
- End-to-end workflow creation and export process
- Tool configuration for agent blocks (MCP, native, database)
- Tool deduplication to prevent "Tool names must be unique" errors
- Variable reference gotchas (snake_case conversion, correct field paths)
- Extending exported services with custom database tools
- Updated troubleshooting section with common issues and fixes

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

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: aadamsx