v0.20.0

Upgrades the laravel/ai dependency to the v0.9 line. This is the first Swarm
release to require laravel/ai ^0.9 (v0.9.0); support for the 0.8 line is
dropped. No Swarm source changes were required — the bump is verified against
the full test suite (1919 passing) and PHPStan level 7 — but because Swarm does
not isolate your application from Laravel AI, treat it as an integration-test
event. See UPGRADING.md for the checklist.

Changed

• Requires laravel/ai ^0.9 (was ^0.8). laravel/ai v0.9.0 consolidates
  provider tool-calling onto a shared TextGenerationLoop / StepTextGateway,
  switches Anthropic to native structured outputs by default (default text model
  now Claude Sonnet 5), adds collection generics across response and streaming
  events, and introduces new capabilities — filesystem tools for agents, a
  WithoutBroadcasting stream-event attribute, native anyOf structured-output
  schemas, and a configurable OpenAI-compatible provider. None of laravel/ai's
  breaking changes (the removed TextGateway contract; the Anthropic
  default-model change) affect Swarm's integration surface — the removed contract
  is not referenced by Swarm, and no default model is pinned or asserted.