Converge Pack

This crate is the strict Rust authoring contract for Converge packs. External modules implement these traits to participate in convergence:

• [Suggestor] for pure suggestors
• [Context] for read-only context access
• [AgentEffect] for buffered proposal output
• [Fact] / [ProposedFact] for the current context boundary

Provider selection and backend capability routing do not live here. Those contracts belong to converge-provider-api.