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/config/zod-schema.ts

@@ -103,6 +103,14 @@ const BlockStreamingCoalesceSchema = z.object({
   idleMs: z.number().int().nonnegative().optional(),
 });
 
+const HumanDelaySchema = z.object({
+  mode: z
+    .union([z.literal("off"), z.literal("natural"), z.literal("custom")])
+    .optional(),
+  minMs: z.number().int().nonnegative().optional(),
+  maxMs: z.number().int().nonnegative().optional(),
+});
+
 const normalizeAllowFrom = (values?: Array<string | number>): string[] =>
   (values ?? []).map((v) => String(v).trim()).filter(Boolean);
 
@@ -775,6 +783,7 @@ const AgentEntrySchema = z.object({
   workspace: z.string().optional(),
   agentDir: z.string().optional(),
   model: z.string().optional(),
+  humanDelay: HumanDelaySchema.optional(),
   identity: IdentitySchema,
   groupChat: GroupChatSchema,
   subagents: z
@@ -1043,6 +1052,7 @@ const AgentDefaultsSchema = z
       })
       .optional(),
     blockStreamingCoalesce: BlockStreamingCoalesceSchema.optional(),
+    humanDelay: HumanDelaySchema.optional(),
     timeoutSeconds: z.number().int().positive().optional(),
     mediaMaxMb: z.number().positive().optional(),
     typingIntervalSeconds: z.number().int().positive().optional(),
@@ -1089,7 +1099,6 @@ const AgentDefaultsSchema = z
       .optional(),
   })
   .optional();
-
 export const ClawdbotSchema = z
   .object({
     env: z