feat(agent): split queue into steer() and followUp() APIs

Breaking change: replaces queueMessage() with two separate methods:
- steer(msg): interrupt mid-run, delivered after current tool execution
- followUp(msg): wait until agent finishes before delivery

Also renames:
- queueMode -> steeringMode/followUpMode
- getQueuedMessages -> getSteeringMessages/getFollowUpMessages

Refs #403

packages/agent/test/agent-loop.test.ts

@@ -340,8 +340,8 @@ describe("agentLoop with AgentMessage", () => {
 		const config: AgentLoopConfig = {
 			model: createModel(),
 			convertToLlm: identityConverter,
-			getQueuedMessages: async () => {
-				// Return queued message after first tool executes
+			getSteeringMessages: async () => {
+				// Return steering message after first tool executes
 				if (executed.length === 1 && !queuedDelivered) {
 					queuedDelivered = true;
 					return [queuedUserMessage];