Add examples/claw — E2E example for ADK Elixir (#83)

[zeroasterisk]

What

examples/claw/ — a minimal but real AI assistant (mini OpenClaw) that exercises the full ADK Elixir stack end-to-end.

Features demonstrated

Feature	Implementation
Multi-agent	Router agent + coder + helper sub-agents
Tool use	3 tools: datetime, read_file, shell_command (sandboxed)
Session persistence	JsonFile store (priv/sessions/)
Callbacks	Claw.Callbacks — logs all LLM calls
A2A server	Available at /a2a
Phoenix LiveView	Chat UI at / using ADK.Phoenix.ChatLive
Model	gemini-2.0-flash-lite (cheapest)

Status

• ✅ Compiles cleanly
• ✅ 10 tests, all passing
• ✅ README with setup instructions

ADK Elixir feedback (found while building)

See README.md for detailed notes. Key findings:

1. No ReflectRetry plugin exists yet
2. Session store config is global (can't vary per runner)
3. ChatLive uses deprecated phx-update="append" — should use streams
4. Runner doesn't pass session_store through to sessions
5. Sub-agent routing/delegation not yet wired up automatically
6. A2A Server creates new ETS table on every init/1
7. Anonymous fns in tools can't be used in compile-time plug config (had to wrap A2A in a controller)

Closes #83