feat(smtp): SMTP egress provider (nodemailer) for the EMAIL channel

Makes external email actually leave the building (was sandbox-only). SmtpProvider
implements CapabilityProvider; env-activated (accounts@ primary, ceo@ fallback);
transporter injected for tests.

- send() maps target+payload -> {from,to,subject,html,text,inReplyTo,references};
  providerRef = nodemailer's real Message-ID (so replies thread via In-Reply-To).
- Fallback ONLY on pre-acceptance failures (connect/auth/timeout) — a post-acceptance
  error is terminal, so a message the server already took can't be double-delivered.
- Never throws — transport failure -> FAILED, per the adapter doctrine.
- Registry precedence via registration order: SMTP > HTTP relay > sandbox for EMAIL.

Verified: 13 unit tests (config/envelope/fallback/precedence) + a REAL SMTP round-trip
against nodemailer Ethereal (SENT, genuine Message-ID). Full suite 281/281, boundary+build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -0,0 +1,34 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { CapabilityProviderRegistry } from './capability.registry';
+
+// The registry reads env in its constructor, so each case sets env → constructs a fresh registry →
+// asserts → restores env.
+const SMTP_KEYS = ['IIOS_SMTP_HOST', 'IIOS_SMTP_USER', 'IIOS_SMTP_PASS', 'IIOS_PROVIDER_URL_EMAIL'];
+const saved: Record<string, string | undefined> = {};
+function set(env: Record<string, string | undefined>) {
+  for (const k of SMTP_KEYS) { saved[k] = process.env[k]; delete process.env[k]; }
+  for (const [k, v] of Object.entries(env)) if (v != null) process.env[k] = v;
+}
+afterEach(() => { for (const k of SMTP_KEYS) { if (saved[k] == null) delete process.env[k]; else process.env[k] = saved[k]; } });
+
+describe('CapabilityProviderRegistry — EMAIL precedence (SMTP > HTTP > sandbox)', () => {
+  it('binds SMTP for EMAIL when the SMTP env trio is set', () => {
+    set({ IIOS_SMTP_HOST: 'smtp.test', IIOS_SMTP_USER: 'accounts@x', IIOS_SMTP_PASS: 'p' });
+    expect(new CapabilityProviderRegistry().forChannel('EMAIL').name).toBe('smtp');
+  });
+
+  it('binds the HTTP EmailProvider when only IIOS_PROVIDER_URL_EMAIL is set', () => {
+    set({ IIOS_PROVIDER_URL_EMAIL: 'https://relay.test/send' });
+    expect(new CapabilityProviderRegistry().forChannel('EMAIL').name).toBe('email-http');
+  });
+
+  it('SMTP wins over the HTTP relay when both are set', () => {
+    set({ IIOS_SMTP_HOST: 'smtp.test', IIOS_SMTP_USER: 'accounts@x', IIOS_SMTP_PASS: 'p', IIOS_PROVIDER_URL_EMAIL: 'https://relay.test/send' });
+    expect(new CapabilityProviderRegistry().forChannel('EMAIL').name).toBe('smtp');
+  });
+
+  it('falls back to the sandbox when neither is configured', () => {
+    set({});
+    expect(new CapabilityProviderRegistry().forChannel('EMAIL').name).toBe('sandbox');
+  });
+});