Fix branch fallback in add-context modal to use session branch

When the branch input is empty, now defaults to sessions/${sessionName}
instead of 'main' when adding a repository.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: sallyom

components/frontend/src/app/projects/[name]/sessions/[sessionName]/components/modals/add-context-modal.tsx

@@ -34,7 +34,8 @@ export function AddContextModal({
   const handleSubmit = async () => {
     if (!contextUrl.trim()) return;
 
-    await onAddRepository(contextUrl.trim(), contextBranch.trim() || 'main', autoPush);
+    const defaultBranch = sessionName ? `sessions/${sessionName}` : 'main';
+    await onAddRepository(contextUrl.trim(), contextBranch.trim() || defaultBranch, autoPush);
 
     // Reset form
     setContextUrl("");