Agentic Context Engine Files

Added

• Usage metering hook — RecursiveConfig.usage_callback: (RequestUsage, model_id) -> None fires once per pydantic-ai model request (orchestrator turns, sub-agent runs, tool-call follow-ups). Implemented via ace.rr.MeteredModel, a pydantic_ai.models.wrapper.WrapperModel subclass, so metering lives at the framework's own model boundary — one firing site, no per-call-site plumbing. Callback exceptions are caught and logged so metering never crashes the pipeline.
• Pre-built model instance support — RRStep, create_rr_agent, create_sub_agent, and RecursiveConfig.subagent_model now accept either a model-id string or a pre-built pydantic_ai.models.Model instance. Enables callers that need a custom provider (e.g. a Bedrock model carrying STS-assumed credentials) to inject a fully-configured model rather than resolving from a string.
• Sub-agent model_settings — create_sub_agent now threads an explicit ModelSettings parameter into its PydanticAgent constructor.

Back-compat

Existing RRStep(model="...") callers are unchanged. The widened type signatures are additive.