feat(agent): add human-like delay between block replies

Adds `agent.humanDelay` config option to create natural rhythm between
streamed message bubbles. When enabled, introduces a random delay
(default 800-2500ms) between block replies, making multi-message
responses feel more like natural human texting.

Config example:
```json
{
  "agent": {
    "blockStreamingDefault": "on",
    "humanDelay": {
      "enabled": true,
      "minMs": 800,
      "maxMs": 2500
    }
  }
}
```

- First message sends immediately
- Subsequent messages wait a random delay before sending
- Works with iMessage, Signal, and Discord providers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/gateway/configuration-examples.md

@@ -218,6 +218,9 @@ Save to `~/.clawdbot/clawdbot.json` and you can DM the bot from that number.
       blockStreamingCoalesce: {
         idleMs: 1000
       },
+      humanDelay: {
+        mode: "natural"
+      },
       timeoutSeconds: 600,
       mediaMaxMb: 5,
       typingIntervalSeconds: 5,