docs: document native audio support across README, CHANGELOG, config, and planning docs

- README: add audio.transcribe to tool list, update media pipeline description,
  add Native Audio Support and Audio Transcription config sections, add
  supports_audio per-tier override example
- SOUL.md: add audio.transcribe to available tools list
- CHANGELOG: add native audio support and audio.transcribe tool entries
- config/default.yaml: add commented audio config section, supports_audio hint
- INTEGRATIONS.md: expand audio section with native passthrough, capabilities,
  smart routing, AudioSource type, token estimation, audio.transcribe tool
- STRUCTURE.md: add capabilities.ts and audio-transcribe.ts to key file listings
- ARCHITECTURE.md: update data flow step 5 to describe smart audio routing

.planning/codebase/ARCHITECTURE.md

@@ -161,7 +161,7 @@
 2. Adapter calls `onMessage()` callback → `ChannelRegistry.handleInbound()` routes to `MessageHandler`
 3. `createMessageRouter()` resolves agent config via `AgentRouter.resolve(channel, senderId)`
 4. `getOrCreateAgent()` creates/retrieves `AgentOrchestrator` for the session (cached by `channel:sender:agentConfig`)
-5. Audio attachments transcribed if present
+5. Audio routing: `supportsAudioInput()` checks provider capability — native audio passed through for Gemini/OpenAI/GitHub, transcribed via Whisper for others
 6. `orchestrator.process()` → injects memory context → checks compaction → delegates to `NativeAgent.process()`
 7. `NativeAgent.toolLoop()` → sends to `ModelRouter.chat()` → model returns response or tool calls
 8. If tool calls: `ToolExecutor.execute()` → policy check → hook check → tool execution → loop back to model