docs: add custom agents & sub-agent orchestration guide

Add comprehensive documentation for custom agents covering:
- Defining custom agents with scoped tools and prompts
- Configuration reference for all CustomAgentConfig properties
- How sub-agent delegation and inference work
- Listening to sub-agent lifecycle events (started/completed/failed/selected)
- Building agent tree UIs from event data
- Attaching MCP servers to agents
- Best practices and common patterns

Code examples provided for all four SDK languages (TypeScript, Python, Go, .NET).

Addresses #619 and #301.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: patniko