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/concepts/messages.md

@@ -61,6 +61,7 @@ Key settings:
 - `agents.defaults.blockStreamingBreak` (`text_end|message_end`)
 - `agents.defaults.blockStreamingChunk` (`minChars|maxChars|breakPreference`)
 - `agents.defaults.blockStreamingCoalesce` (idle-based batching)
+- `agents.defaults.humanDelay` (human-like pause between block replies)
 - Provider overrides: `*.blockStreaming` and `*.blockStreamingCoalesce` (non-Telegram providers require explicit `*.blockStreaming: true`)
 
 Details: [Streaming + chunking](/concepts/streaming).