Add PS_AI_Agent_ElevenLabs plugin (initial implementation)

Adds a new UE5.5 plugin integrating the ElevenLabs Conversational AI Agent
via WebSocket. No gRPC or third-party libs required.

Plugin components:
- UElevenLabsSettings: API key + Agent ID in Project Settings
- UElevenLabsWebSocketProxy: full WS session lifecycle, JSON message handling,
  ping/pong keepalive, Base64 PCM audio send/receive
- UElevenLabsConversationalAgentComponent: ActorComponent for NPC voice
  conversation, orchestrates mic capture -> WS -> procedural audio playback
- UElevenLabsMicrophoneCaptureComponent: wraps Audio::FAudioCapture,
  resamples to 16kHz mono, dispatches on game thread

Also adds .claude/ memory files (project context, plugin notes, patterns)
so Claude Code can restore full context on any machine after a git pull.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/elevenlabs_plugin.md

@@ -0,0 +1,61 @@
+# PS_AI_Agent_ElevenLabs Plugin
+
+## Location
+`Unreal/PS_AI_Agent/Plugins/PS_AI_Agent_ElevenLabs/`
+
+## File Map
+```
+PS_AI_Agent_ElevenLabs.uplugin
+Source/PS_AI_Agent_ElevenLabs/
+  PS_AI_Agent_ElevenLabs.Build.cs
+  Public/
+    PS_AI_Agent_ElevenLabs.h                  – FPS_AI_Agent_ElevenLabsModule + UElevenLabsSettings
+    ElevenLabsDefinitions.h                   – Enums, structs, ElevenLabsMessageType/Audio constants
+    ElevenLabsWebSocketProxy.h/.cpp           – UObject managing one WS session
+    ElevenLabsConversationalAgentComponent.h/.cpp  – Main ActorComponent (attach to NPC)
+    ElevenLabsMicrophoneCaptureComponent.h/.cpp    – Mic capture, resample, dispatch to game thread
+  Private/
+    (implementations of the above)
+```
+
+## ElevenLabs Conversational AI Protocol
+- **WebSocket URL**: `wss://api.elevenlabs.io/v1/convai/conversation?agent_id=<ID>`
+- **Auth**: HTTP upgrade header `xi-api-key: <key>` (set in Project Settings)
+- **All frames**: JSON text (no binary frames used by the API)
+- **Audio format**: PCM 16-bit signed, 16000 Hz, mono, little-endian — Base64-encoded in JSON
+
+### Client → Server messages
+| Type field value | Payload |
+|---|---|
+| *(none – key is the type)* `user_audio_chunk` | `{ "user_audio_chunk": "<base64 PCM>" }` |
+| `user_turn_start` | `{ "type": "user_turn_start" }` |
+| `user_turn_end` | `{ "type": "user_turn_end" }` |
+| `interrupt` | `{ "type": "interrupt" }` |
+| `pong` | `{ "type": "pong", "pong_event": { "event_id": N } }` |
+
+### Server → Client messages (field: `type`)
+| type value | Key nested object | Notes |
+|---|---|---|
+| `conversation_initiation_metadata` | `conversation_initiation_metadata_event.conversation_id` | Marks WS ready |
+| `audio` | `audio_event.audio_base_64` | Base64 PCM from agent |
+| `transcript` | `transcript_event.{speaker, message, is_final}` | User or agent speech |
+| `agent_response` | `agent_response_event.agent_response` | Final agent text |
+| `interruption` | — | Agent stopped mid-sentence |
+| `ping` | `ping_event.event_id` | Must reply with pong |
+
+## Key Design Decisions
+- **No gRPC / no ThirdParty libs** — pure UE WebSockets + HTTP, builds out of the box
+- Audio resampled in-plugin: device rate → 16000 Hz mono (linear interpolation)
+- `USoundWaveProcedural` for real-time agent audio playback (queue-driven)
+- Silence heuristic: 30 game-thread ticks (~0.5 s at 60 fps) with no new audio → agent done speaking
+- `bSignedURLMode` setting: fetch a signed WS URL from your own backend (keeps API key off client)
+- Two turn modes: `Server VAD` (ElevenLabs detects speech end) and `Client Controlled` (push-to-talk)
+
+## Build Dependencies (Build.cs)
+Core, CoreUObject, Engine, InputCore, Json, JsonUtilities, WebSockets, HTTP,
+AudioMixer, AudioCaptureCore, AudioCapture, Voice, SignalProcessing
+
+## Status
+- **Session 1** (2026-02-19): All source files written, registered in .uproject. Not yet compiled.
+- **TODO**: Open in UE 5.5 Editor → compile → test basic WS connection with a test agent ID.
+- **Watch out**: Verify `USoundWaveProcedural::OnSoundWaveProceduralUnderflow` delegate signature vs UE 5.5 API.