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.slack.example

@@ -0,0 +1,23 @@
+# Slack Transport
+# Copy this file to bot.toml and fill in your values.
+# Only one transport can be active at a time.
+#
+# Setup:
+#   1. Create a Slack App at api.slack.com/apps
+#   2. Add OAuth scopes: chat:write, chat:update
+#   3. Subscribe to bot events: message.channels, message.groups, message.im
+#   4. Install the app to your workspace
+#   5. Set your webhook URL in Event Subscriptions: https://your-server/webhook/slack
+
+enabled = true
+transport = "slack"
+
+slack_bot_token = "xoxb-..."
+slack_signing_secret = "your-signing-secret"
+slack_channel_ids = ["C01ABCDEF"]
+
+# Bot display name (used in formatted messages).
+# display_name = "Assistant"
+
+# Maximum conversation turns to remember per channel (default: 20).
+# history_size = 20