feat(coding-agent): update AgentSession for steer()/followUp() API

- Rename queueMessage to steer(), add followUp()
- Split _pendingMessages into _steeringMessages and _followUpMessages
- Update sendHookMessage to accept deliverAs option
- Rename hasQueuedMessages to hasPendingMessages
- Rename queuedMessageCount to pendingMessageCount
- Update clearQueue() return type to { steering, followUp }
- Update UI to show steering vs follow-up messages differently

WIP: settings-manager, sdk, interactive-mode, rpc-mode still need updates

packages/coding-agent/src/core/sdk.ts

@@ -575,7 +575,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		modelRegistry,
 		model: agent.state.model,
 		isIdle: () => !session.isStreaming,
-		hasQueuedMessages: () => session.queuedMessageCount > 0,
+		hasPendingMessages: () => session.pendingMessageCount > 0,
 		abort: () => {
 			session.abort();
 		},