//! Agent variants — different strategies for LLM ↔ tool interaction.
//!
//! - `sgr` — structured output via discriminated union schema
//! - `tool_calling` — native function calling (Gemini FC / OpenAI tools)
//! - `flexible` — text-based with retry + error feedback
//! - `hybrid` — 2-phase: reasoning FC → action FC
//! - `planning` — read-only wrapper that produces structured plans
//! - `clarification` — built-in system tools (ClarificationTool, PlanTool)