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/secrets.mjs

@@ -16,8 +16,8 @@ export function load_secrets(file_path) {
 	} catch (err) {
 		throw new Error(`Secrets file is not valid JSON: ${err.message}`);
 	}
-	if (!parsed.users || typeof parsed.users !== "object") {
+	if (!parsed.users || typeof parsed.users !== 'object') {
 		throw new Error("Secrets file must have a top-level 'users' object");
 	}
-	return parsed;
+	return parsed; // callers destructure { users, smtp }
 }