feat: default to allowlist for dedicated numbers in onboarding (#100)

When user selects "dedicated bot number" mode (selfChatMode=false),
skip the dmPolicy question and default to allowlist. Prompt for
allowed phone numbers immediately.

This is simpler and safer than pairing mode, which sends codes to
whoever messages the bot.

Users who want pairing or open mode can edit lettabot.yaml manually.

Also updates docs to reflect the new defaults.

Written by Cameron ◯ Letta Code

"Simplicity is the ultimate sophistication." - Leonardo da Vinci

docs/configuration.md

@@ -117,8 +117,12 @@ All channels share these common options:
 
 ### DM Policies
 
-- **`pairing`** (recommended): New users get a code, approve with `lettabot pairing approve`
-- **`allowlist`**: Only specified user IDs can message
+**Note:** For WhatsApp/Signal with `selfChat: true` (personal number), dmPolicy is ignored - only you can message via "Message Yourself" / "Note to Self".
+
+For dedicated bot numbers (`selfChat: false`), onboarding defaults to **allowlist**:
+
+- **`allowlist`** (default for dedicated numbers): Only specified phone numbers can message
+- **`pairing`**: New users get a code, approve with `lettabot pairing approve`
 - **`open`**: Anyone can message (not recommended)
 
 ### Channel-Specific Options