docs: comprehensive documentation audit and improvements (#90)

Fixes and updates:
- README.md: Remove duplicate heartbeat troubleshooting section
- docs/getting-started.md: Fix Node version (18→20), commands, repo URL
- docs/commands.md: Rewrite with accurate command list (/start, /status, /heartbeat)
- docs/README.md: New multi-channel architecture diagram
- docs/whatsapp-setup.md: Add selfChatMode safety docs, media support section
- docs/slack-setup.md: Fix broken links

New documentation:
- docs/configuration.md: Complete YAML config reference
- docs/cron-setup.md: Scheduling guide (cron jobs + heartbeats)

Written by Cameron ◯ Letta Code

"Documentation is a love letter that you write to your future self." - Damian Conway

docs/getting-started.md

@@ -4,7 +4,7 @@ Get LettaBot running in 5 minutes.
 
 ## Prerequisites
 
-- Node.js 18+ 
+- Node.js 20+ 
 - npm or yarn
 - A Telegram account
 - A Letta account ([app.letta.com](https://app.letta.com))
@@ -14,7 +14,7 @@ Get LettaBot running in 5 minutes.
 ### 1. Clone and Install
 
 ```bash
-git clone https://github.com/yourusername/lettabot.git
+git clone https://github.com/letta-ai/lettabot.git
 cd lettabot
 npm install
 ```
@@ -37,7 +37,9 @@ npm install
 **Option A: Interactive Setup (Recommended)**
 
 ```bash
-npm run setup
+npm run build
+npm link
+lettabot onboard
 ```
 
 This will walk you through configuration interactively.
@@ -97,4 +99,5 @@ ALLOWED_USERS=123456789,987654321
 
 - [Commands Reference](./commands.md) - Learn all bot commands
 - [Gmail Integration](./gmail-pubsub.md) - Set up email notifications
-- Check out [PLAN.md](../PLAN.md) for the full roadmap
+- [Slack Setup](./slack-setup.md) - Add Slack channel
+- [Discord Setup](./discord-setup.md) - Add Discord channel