docs: phase 02 completion — voice/audio system docs + architecture updates

Update architecture overview with streaming TTS provider layer. Expand tools
system docs with voice/model resolution. Update HTTP and WebSocket RPC
documentation for voice endpoints. Record Phase 02 completion in changelog.
Update CLAUDE.md with voice picker and streaming TTS patterns.

docs/00-architecture-overview.md

@@ -475,6 +475,29 @@ Six distinct workspace scenarios:
 - **L0 Auto-Injection**: Top-K vault entries injected into system prompt as "relevant context from vault"
 - **Filesystem Sync**: Vault entries exported as `.md` files for manual editing, re-imported with change tracking
 
+### Audio & Voice System (ElevenLabs + Streaming TTS)
+
+**Provider architecture** (`internal/audio/`):
+- **`Manager`**: Central orchestrator dispatching TTS/STT/Music/SFX requests to pluggable providers
+- **`TTSProvider` interface**: Core text-to-speech contract (blocking, buffered response)
+- **`StreamingTTSProvider` interface**: Optional interface for ElevenLabs `/stream` endpoint (chunked audio via `io.ReadCloser`)
+- **Implementations**: ElevenLabs, OpenAI, Edge, MiniMax (phase-gated; STT/Music/SFX partial)
+
+**Voice discovery & caching**:
+- **Voice cache** (`internal/audio/voice_cache.go`): In-memory LRU (cap 1000 tenants, TTL 1h) shared by HTTP `/v1/voices` + WS `voices.list` handlers. Thread-safe with `sync.Mutex` (LRU updates require write lock)
+- **Cache miss recovery**: HTTP handler auto-fetches from ElevenLabs; WS handler requires prior cache warm (provider resolution deferred to Phase 3)
+- **Agent audio context** (`store.WithAgentAudio` / `AgentAudioFromCtx`): Immutable snapshot bundle (AgentID + OtherConfig JSONB) injected by dispatcher before tool dispatch; consumed by `TtsTool.Execute` for voice/model resolution
+
+**Agent-level configuration** (`agents.other_config` JSONB):
+- `tts_voice_id`: ElevenLabs voice ID (e.g., "pMsXgVXv3BLzUgSXRplE")
+- `tts_model_id`: Model choice (eleven_v3, eleven_flash_v2_5, eleven_multilingual_v2, eleven_turbo_v2_5)
+- Resolution precedence: CLI args → agent config → tenant override → provider default
+
+**Web UI voice picker** (`ui/web/src/components/voice-picker.tsx`):
+- Combobox with BM25 search, preview playback button (HTML `<audio>`)
+- Handles preview CDN 403 (expiry) via `onError` → auto-refresh cache
+- Embedded in PromptSettingsSection, bound to `other_config.tts_voice_id`
+
 ---
 
 ## Cross-References