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/imessage/monitor.ts

@@ -1,4 +1,7 @@
-import { resolveEffectiveMessagesConfig } from "../agents/identity.js";
+import {
+  resolveEffectiveMessagesConfig,
+  resolveHumanDelayConfig,
+} from "../agents/identity.js";
 import { chunkText, resolveTextChunkLimit } from "../auto-reply/chunk.js";
 import { hasControlCommand } from "../auto-reply/command-detection.js";
 import { formatAgentEnvelope } from "../auto-reply/envelope.js";
@@ -444,6 +447,7 @@ export async function monitorIMessageProvider(
     const dispatcher = createReplyDispatcher({
       responsePrefix: resolveEffectiveMessagesConfig(cfg, route.agentId)
         .responsePrefix,
+      humanDelay: resolveHumanDelayConfig(cfg, route.agentId),
       deliver: async (payload) => {
         await deliverReplies({
           replies: [payload],