docs: split bot.toml.example into per-transport example files

Replace the monolithic bot.toml.example with separate files for each
transport: matrix, whatsapp-meta, whatsapp-twilio, and slack. Add a
chat bot configuration section to the README explaining that only one
transport can be active at a time and how to set up each one.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.storkit/bot.toml.whatsapp-twilio.example

@@ -0,0 +1,24 @@
+# WhatsApp Transport (Twilio)
+# Copy this file to bot.toml and fill in your values.
+# Only one transport can be active at a time.
+#
+# Setup:
+#   1. Sign up at twilio.com
+#   2. Activate the WhatsApp sandbox (Messaging > Try it out > Send a WhatsApp message)
+#   3. Send the sandbox join code from your WhatsApp to the sandbox number
+#   4. Copy your Account SID, Auth Token, and sandbox number below
+#   5. Set your webhook URL in the Twilio console: https://your-server/webhook/whatsapp
+
+enabled = true
+transport = "whatsapp"
+whatsapp_provider = "twilio"
+
+twilio_account_sid = "ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+twilio_auth_token = "your_auth_token"
+twilio_whatsapp_number = "+14155238886"
+
+# Bot display name (used in formatted messages).
+# display_name = "Assistant"
+
+# Maximum conversation turns to remember per user (default: 20).
+# history_size = 20