Update an agent's runtime harness and default model

Updates agents.harness_provider and agent-scoped runtime config. The settings apply to future provider sessions. For model, reasoning_effort, and claude.transport: omit the field to leave it unchanged, send null to clear the corresponding override, or send a value to set it. A model the model catalog does not list is a 400 unless allow_custom_model is true. A latest: alias is not accepted here (aliases resolve for a task's model and for MODEL_TIER_* values); reasoning_effort must be one the harness and model support. profile sets the agent's default model profile (same tri-state): it must exist and run on harness_provider, and it cannot be sent with model. Sending model, or changing the model or effort override by any path, clears the default profile.