docs: add a multi-agent-handoff example and guide

## Context AgentDeck coordinates multi-agent systems where a triage agent hands off conversational control to specialized agents. Handoffs are declared on agents and tracked in the run event log. ## Current state - Agent handoffs are supported by the engine adapter and event schema. - Shipped examples under `examples/` only cover single-agent setups (`chat-agent-with-a-tool`, `agent-with-a-skill`). - There is no isolated, runnable project showing how to declare and execute agent handoffs. ## What is missing A minimal worked example in `examples/multi-agent-handoff/` (`run.py`, `.agentdeck/`, `README.md`) and a matching guide page in `docs-site/content/guides/multi-agent-handoffs.mdx` demonstrating a FrontDesk agent routing queries to a Specialist agent. ## Expected contribution 1. `examples/multi-agent-handoff/run.py` defining a triage agent that hands off to a specialist agent on a single Deck. 2. `examples/multi-agent-handoff/README.md` explaining the handoff flow and showing the resulting event stream. 3. `docs-site/content/guides/multi-agent-handoffs.mdx` documenting handoff patterns and inspection. ## Relevant APIs and source files - `agentdeck/authoring/agent.py` - `Agent` authoring and handoff declarations - `agentdeck/deck.py` - `Deck.run()` and `Deck.stream()` - `examples/chat-agent-with-a-tool/` - reference example structure ## Files to change - `examples/multi-agent-handoff/` (new directory) - `docs-site/content/guides/multi-agent-handoffs.mdx` (new guide page) ## Acceptance criteria - [ ] Uses only public APIs from `agentdeck` - [ ] Any code shown runs against the current release - [ ] Example runs via `python run.py` - [ ] Event stream output demonstrates handoff transitions - [ ] Includes expected terminal output in `README.md` - [ ] Does not duplicate the Quickstart - [ ] Links the related reference page - [ ] `make check` passes - [ ] `docs-site` builds ## Out of scope - No API redesign - No new runtime behavior - No changes to engine adapters ## How to verify locally ```bash make install make check cd examples/multi-agent-handoff && python run.py cd docs-site npm install npm run dev ``` ## Difficulty small