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/test/agent-session-concurrent.test.ts

@@ -144,7 +144,7 @@ describe("AgentSession concurrent prompt guard", () => {
 
 		// queueMessage should work while streaming
 		expect(() => session.queueMessage("Queued message")).not.toThrow();
-		expect(session.queuedMessageCount).toBe(1);
+		expect(session.pendingMessageCount).toBe(1);
 
 		// Cleanup
 		await session.abort();