feat(channels): add DingTalk channel via Stream Mode

Implement DingTalk messaging channel using the official Stream Mode
WebSocket protocol with per-message session webhook replies.

- Add DingTalkChannel with send/listen/health_check support
- Add DingTalkConfig (client_id, client_secret, allowed_users)
- Integrate with onboard wizard, integrations registry, and channel
  list/doctor commands
- Include unit tests for user allowlist rules and config serialization

src/integrations/registry.rs

@@ -125,6 +125,18 @@ pub fn all_integrations() -> Vec<IntegrationEntry> {
             category: IntegrationCategory::Chat,
             status_fn: |_| IntegrationStatus::ComingSoon,
         },
+        IntegrationEntry {
+            name: "DingTalk",
+            description: "DingTalk Stream Mode (钉钉)",
+            category: IntegrationCategory::Chat,
+            status_fn: |c| {
+                if c.channels_config.dingtalk.is_some() {
+                    IntegrationStatus::Active
+                } else {
+                    IntegrationStatus::Available
+                }
+            },
+        },
         // ── AI Models ───────────────────────────────────────────
         IntegrationEntry {
             name: "OpenRouter",