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>

src/slack/monitor.ts

@@ -7,6 +7,7 @@ import type { WebClient as SlackWebClient } from "@slack/web-api";
 import {
   resolveAckReaction,
   resolveEffectiveMessagesConfig,
+  resolveHumanDelayConfig,
 } from "../agents/identity.js";
 import {
   chunkMarkdownText,
@@ -1109,6 +1110,7 @@ export async function monitorSlackProvider(opts: MonitorSlackOpts = {}) {
       createReplyDispatcherWithTyping({
         responsePrefix: resolveEffectiveMessagesConfig(cfg, route.agentId)
           .responsePrefix,
+        humanDelay: resolveHumanDelayConfig(cfg, route.agentId),
         deliver: async (payload) => {
           const effectiveThreadTs = resolveSlackThreadTs({
             replyToMode,