feat(iios): register EmailAdapter for EMAIL + email-shaped egress provider

AdapterRegistry now backs the EMAIL channel with the email-native EmailAdapter
(RFC threading) instead of the generic WebhookAdapter. Adds EmailProvider — an
email-envelope egress provider ({to,subject,text,html,inReplyTo}) registered for
EMAIL when IIOS_PROVIDER_URL_EMAIL is set (sandbox stays the default). Spec:
signed email → normalized interaction on the email thread; a reply joins the
same thread (one conversation); bad signature → rejected (fail-closed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/iios-service/src/capability/capability.registry.ts

@@ -2,6 +2,7 @@ import { Injectable, NotFoundException } from '@nestjs/common';
 import type { CapabilityProvider } from '@insignia/iios-contracts';
 import { SandboxProvider } from './sandbox.provider';
 import { HttpProvider } from './http.provider';
+import { EmailProvider } from './email.provider';
 
 const DEFAULT_CHANNELS = ['WEBHOOK', 'EMAIL', 'WHATSAPP', 'PORTAL'];
 
@@ -19,7 +20,9 @@ export class CapabilityProviderRegistry {
     for (const ch of DEFAULT_CHANNELS) this.register(new SandboxProvider([ch]));
     for (const ch of DEFAULT_CHANNELS) {
       const url = process.env[`IIOS_PROVIDER_URL_${ch}`];
-      if (url) this.register(new HttpProvider(ch, url));
+      if (!url) continue;
+      // EMAIL gets an email-shaped envelope provider; other channels use the generic HTTP one.
+      this.register(ch === 'EMAIL' ? new EmailProvider(url) : new HttpProvider(ch, url));
     }
   }