fix(security): lock down inbound DMs by default

docs/signal.md

@@ -51,7 +51,8 @@ You can still run Clawdbot on your own Signal account if your goal is “respond
     httpPort: 8080,
 
     // Who is allowed to talk to the bot (DMs)
-    allowFrom: ["+15557654321"], // your personal number (or "*")
+    dmPolicy: "pairing", // pairing | allowlist | open | disabled
+    allowFrom: ["+15557654321"], // your personal number ("open" requires ["*"])
 
     // Group policy + allowlist
     groupPolicy: "open",
@@ -64,6 +65,10 @@ You can still run Clawdbot on your own Signal account if your goal is “respond
    - Expect `signal.probe.ok=true` and `signal.probe.version`.
 5) DM the bot number from your phone; Clawdbot replies.
 
+## DM pairing
+- Default: `signal.dmPolicy="pairing"` — unknown DM senders get a pairing code.
+- Approve via: `clawdbot pairing approve --provider signal <code>`.
+
 ## “Do I need a separate number?”
 - If you want “I text her and she texts me back”, yes: **use a separate Signal account/number for the bot**.
 - Your personal account can run `signal-cli`, but you can’t self-chat (Signal loop protection; Clawdbot ignores sender==account).