Nika: Declarative Multi-Provider AI Workflow Orchestration

Nika is a declarative workflow engine for AI tasks written in Rust. Workflows are expressed in versioned YAML files using five orthogonal primitives: infer for LLM calls, exec for shell commands, fetch for HTTP extraction, invoke for MCP tool calls, and agent for autonomous multi-turn loops. Nika parses these into a typed AST, resolves a DAG of task dependencies, and executes ready tasks in parallel via Tokio. A 5-layer structured output defense ensures schema-validated JSON across 7 cloud LLM providers and local GGUF models.