Implement email support with per-user permission model (closes #2)

New modules:
- server/mailer.mjs: nodemailer transport wrapper
- server/mail_perms.mjs: runtime permission store, persisted to disk

New actions:
- send-email: checks (caller, to, topic) permission before sending
- set-mail-permission: grant/revoke permissions, gated by canApprove
- get-mail-permissions: list current permissions

Handler signature extended to handler(params, ctx) where ctx carries
caller, users, mail_perm_store and mailer_send. Existing handlers
ignore ctx so the change is backwards-compatible.

SMTP config lives in secrets.json under optional 'smtp' key.
Mail permissions path via --mail-perms or CONDUIT_MAIL_PERMS.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/mailer.mjs

@@ -0,0 +1,18 @@
+import nodemailer from 'nodemailer';
+
+// Returns an async send(to, subject, body) function.
+// If smtp_config is absent, returns a stub that always throws.
+export function create_mailer(smtp_config) {
+	if (!smtp_config) {
+		return async () => {
+			throw new Error('SMTP not configured');
+		};
+	}
+
+	const { from, ...transport_config } = smtp_config;
+	const transporter = nodemailer.createTransport(transport_config);
+
+	return async function send(to, subject, body) {
+		await transporter.sendMail({ from, to, subject, text: body });
+	};
+}