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>

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New Features and Changes
 - Onboarding/Models: add first-class Z.AI (GLM) auth choice (`zai-api-key`) + `--zai-api-key` flag.
+- Agents: add human-delay pacing between block replies (per-agent configurable). (#446) — thanks @tony-freedomology.
 
 ### Fixes
 - Agents/OpenAI: fix Responses tool-only → follow-up turn handling (avoid standalone `reasoning` items that trigger 400 “required following item”).