v0.18.1

Added

• Docs recipe: Delegating to sub-agents — wire the
  pydantic-ai-harness SubAgents capability through the existing
  CAPABILITIES seam (the [harness] extra) to give the agent a
  delegate_task tool over a roster of named child agents. Zero new package
  code — a stateless capability adopted like CodeMode; covered by an
  importorskip-guarded recipe test. Per-delegate limits (usage_limits /
  timeout_seconds / max_calls / on_failure) are fields on SubAgent.