docs: define customer release branch policy

backend/workflows/templates/workflows/developer_handbook.html

@@ -79,6 +79,7 @@
         <li><code>develop</code> is the active integration branch.</li>
         <li><code>main</code> is the stable branch intended for production promotion.</li>
         <li>Feature work should start from <code>develop</code> and merge back into <code>develop</code>.</li>
+        <li>Customer release branches should start from <code>main</code> when a customer must not receive future feature work automatically.</li>
       </ul>
     </div>
     <div class="box">
@@ -93,6 +94,24 @@
         <li>promote <code>develop</code> into <code>main</code> when stable</li>
       </ol>
     </div>
+    <div class="box">
+      <h3>Customer release branches</h3>
+      <p>Use a dedicated release branch when a customer should receive the current stable product line but not future features by default.</p>
+      <ul>
+        <li>Example branch: <code>release/tubco-v1</code></li>
+        <li>Example baseline tag: <code>tubco-baseline-2026-03</code></li>
+        <li>Base the branch on the current <code>main</code>.</li>
+        <li>Continue product work on <code>develop</code>.</li>
+        <li>Only backport approved:
+          <ul>
+            <li>bug fixes</li>
+            <li>security updates</li>
+            <li>UI improvements</li>
+          </ul>
+        </li>
+      </ul>
+      <p>This keeps the customer line stable while the main product keeps evolving.</p>
+    </div>
 
     <h2 id="local">4) Local Development Workflow</h2>
     <h3>Start</h3>