Add tool authoring deep-dive (schema inference, async, error handling)

[AmitoVrito]

Expand docs/agents/tools.md or create a new docs/agents/tool-authoring.md:

• How @tool generates JSON Schema from type hints
• Required vs optional params
• Async tools
• Tools with side effects (databases, APIs) — idempotency advice
• Tools that return structured data
• Tools with state (class-based tools)
• Error handling in tools
• Testing tools in isolation with pytest
• Registering tools in a central registry

[AmitoVrito]

Completed in PR #99 — docs/agents/tool-authoring.md created with @tool decorator, async tools, BaseTool, testing guide.