Agent/AgentState split complete — separate context and state locks

Agent is now Arc<Agent> (immutable config). ContextState and AgentState
have separate tokio::sync::Mutex locks. The parser locks only context,
tool dispatch locks only state. No contention between the two.

All callers migrated: mind/, user/, tools/, oneshot, dmn, learn.
28 tests pass, zero errors.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/oneshot.rs

@@ -14,8 +14,8 @@ use std::fs;
 use std::path::PathBuf;
 use std::sync::OnceLock;
 
-use super::api::{ApiClient, Usage};
-use super::context::{AstNode, Role};
+use super::api::ApiClient;
+use super::context::AstNode;
 use super::tools::{self as agent_tools};
 use super::Agent;