Add voice message transcription support (#54)

* Add voice message transcription support (all channels)

Adds OpenAI Whisper transcription for voice messages across all channels:
- Telegram: ctx.message.voice
- WhatsApp: audioMessage via downloadMediaMessage
- Signal: audio attachments from local files
- Slack: audio files via url_private_download
- Discord: audio attachments

Voice messages sent to agent as "[Voice message]: <transcript>"

Configuration (config takes priority over env):
- lettabot.yaml: transcription.apiKey, transcription.model
- Env: OPENAI_API_KEY, TRANSCRIPTION_MODEL

Closes #47

Written by Cameron ◯ Letta Code

"The best interface is no interface - just talk."

* Add voice message documentation to README

- Add Voice Messages to features list
- Add configuration section for transcription
- Document supported channels

Written by Cameron ◯ Letta Code

* Notify users when voice transcription is not configured

Instead of silently ignoring voice messages, send a helpful message
linking to the documentation.

Written by Cameron ◯ Letta Code

* feat: upgrade to letta-code-sdk main + fix Signal voice transcription

- Switch from published SDK (v0.0.3) to local main branch (file:../letta-code-sdk)
- Update bot.ts for new SDK API: createSession(agentId?, options) signature
- Add conversationId tracking to store for proper conversation persistence
- Fix Signal voice transcription: read attachments from ~/.local/share/signal-cli/attachments/
- Fix Telegram markdown ESM issue: make markdownToTelegramV2 async with dynamic import
- Add transcription config to lettabot.yaml
- Add extensive debug logging for queue and session processing

Signal voice messages now properly transcribe and send to agent.

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: update Signal CLI message sender to use daemon JSON-RPC API

- Switch from signal-cli-rest-api to signal-cli daemon (port 8090)
- Use JSON-RPC send method instead of REST /v2/send
- Support group IDs with group: prefix
- Handle 201 responses and empty bodies correctly

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* Add placeholder for untranscribed voice messages on Signal

If a voice-only message arrives and transcription fails or is disabled,
forward a placeholder so the user knows the message was received.

Written by Cameron ◯ Letta Code

---------

Co-authored-by: Letta <noreply@letta.com>

README.md

@@ -10,6 +10,7 @@ Your personal AI assistant that remembers everything across **Telegram, Slack, D
 - **Unified Memory** - Single agent remembers everything from all channels
 - **Persistent Memory** - Agent remembers conversations across sessions (days/weeks/months)
 - **Local Tool Execution** - Agent can read files, search code, run commands on your machine
+- **Voice Messages** - Automatic transcription via OpenAI Whisper
 - **Heartbeat** - Periodic check-ins where the agent reviews tasks
 - **Scheduling** - Agent can create one-off reminders and recurring tasks
 - **Streaming Responses** - Real-time message updates as the agent thinks
@@ -97,6 +98,31 @@ That's it! Message your bot on Telegram.
 
 > **Note:** For detailed environment variable reference and multi-channel setup, see [SKILL.md](./SKILL.md)
 
+## Voice Messages
+
+LettaBot can transcribe voice messages using OpenAI Whisper. Voice messages are automatically converted to text and sent to the agent with a `[Voice message]:` prefix.
+
+**Supported channels:** Telegram, WhatsApp, Signal, Slack, Discord
+
+### Configuration
+
+Add your OpenAI API key to `lettabot.config.yaml`:
+
+```yaml
+transcription:
+  provider: openai
+  apiKey: sk-...
+  model: whisper-1  # optional, defaults to whisper-1
+```
+
+Or set via environment variable:
+
+```bash
+export OPENAI_API_KEY=sk-...
+```
+
+If no API key is configured, voice messages are silently ignored.
+
 ## Skills
 LettaBot is compatible with [skills.sh](https://skills.sh) and [Clawdhub](https://clawdhub.com/).