test(email): add structural guard for shared email partials

docs/superpowers/plans/2026-06-26-email-templates-brand-hygiene.md

@@ -341,7 +341,7 @@ Cheap catch for future drift, and the **gate** for Tasks 6–9. No test framewor
 **Interfaces:**
 - Produces: a script that exits non-zero if any `email/*.html` (excluding `partials/`) is missing a header IMPORT, footer IMPORT, or `<!-- preheader -->` marker. Run after every template conversion.
 
-- [ ] **Step 1: Write the script**
+- [x] **Step 1: Write the script**
 
 ```js
 #!/usr/bin/env node
@@ -372,12 +372,12 @@ if (failures.length) {
 console.log(`Email template check passed (${files.length} templates).`);
 ```
 
-- [ ] **Step 2: Run it — expect failures for every not-yet-converted template, pass for `reset.html`**
+- [x] **Step 2: Run it — expect failures for every not-yet-converted template, pass for `reset.html`**
 
 Run: `node scripts/check-emails.js`
 Expected: FAIL, listing `welcome.html`, `verify-email.html`, `reset_notify.html`, `registration_accepted.html`, `invitation.html`, `banned.html`, `test.html`, `notification.html`, `digest.html` (reset.html absent from the list). This proves the guard discriminates.
 
-- [ ] **Step 3: Commit**
+- [x] **Step 3: Commit**
 
 ```bash
 git add scripts/check-emails.js