//! Extension system for SynapsCLI.
//!
//! Provides compiled-in hook call sites (`HookBus`) and external extension
//! runtimes that can subscribe to hooks, register tools, and register providers
//! via a stable JSON-RPC 2.0 protocol.
//!
//! # Architecture
//!
//! ```text
//! SynapsCLI binary
//! ├─ HookBus (dispatcher) ← this module
//! ├─ ExtensionManager (lifecycle) ← this module
//! └─ optional external extensions
//! └─ Process/JSON-RPC runtime ← phase 1
//! ```