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/custom-tools/types.ts

@@ -50,7 +50,7 @@ export interface CustomToolContext {
 	/** Whether the agent is idle (not streaming) */
 	isIdle(): boolean;
 	/** Whether there are queued messages waiting to be processed */
-	hasQueuedMessages(): boolean;
+	hasPendingMessages(): boolean;
 	/** Abort the current agent operation (fire-and-forget, does not wait) */
 	abort(): void;
 }