feat: add Unity 6 client with 2.5D visual system

Unity client features:
- WebSocket connection via NativeWebSocket
- 2.5D agent visuals with programmatic placeholder sprites
- Billboard system for sprites and UI elements
- Floating UI panels (name, HP, energy bars)
- Speech bubble system with pop-in animation
- RTS-style camera controller (WASD + scroll zoom)
- Editor tools for prefab creation and scene setup

Scripts:
- NetworkManager: WebSocket singleton
- GameManager: Agent spawning and event handling
- AgentVisual: 2.5D sprite and UI creation
- Billboard: Camera-facing behavior
- SpeechBubble: Animated dialogue display
- CameraController: RTS camera with UI input detection
- UIManager: HUD and command input

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

unity-client/ProjectSettings/AudioManager.asset

@@ -0,0 +1,23 @@
+%YAML 1.1
+%TAG !u! tag:unity3d.com,2011:
+--- !u!11 &1
+AudioManager:
+  m_ObjectHideFlags: 0
+  serializedVersion: 2
+  m_Volume: 1
+  Rolloff Scale: 1
+  Doppler Factor: 1
+  Default Speaker Mode: 2
+  m_SampleRate: 0
+  m_DSPBufferSize: 1024
+  m_VirtualVoiceCount: 512
+  m_RealVoiceCount: 32
+  m_EnableOutputSuspension: 1
+  m_SpatializerPlugin: 
+  m_AmbisonicDecoderPlugin: 
+  m_DisableAudio: 0
+  m_VirtualizeEffects: 1
+  m_RequestedDSPBufferSize: 0
+  m_AudioFoundation: 0
+  m_OutputChannelLayout: 2
+  m_OutputSamplingRate: 48000