fix: handle missing/corrupt repos when fetching branches + clearer manual fallback

Backend (git_repositories.py):
- get_repository_branches now checks for .git subdirectory (not just dir existence)
- If local repo is corrupt/missing but has remote_url, falls back to git ls-remote
  to list branches from the remote
- Returns 404 with actionable message instead of 400 with raw git stderr
- Pre-existing test failure in test_git_repository_clone_preflight.py unchanged

Frontend (workspace-create-form.tsx):
- When branch API fails, auto-switches to manual text input (no dropdown selection needed)
- Shows hint text: 'Couldn't load branches — type one manually'
- useGitRepo hook auto-fetches branches when projectId/repoId change

Quality gates: ruff clean, tsc --noEmit clean, 93 passed + 1 pre-existing failure

apps/web/src/hooks/use-git-repo.ts

@@ -72,7 +72,11 @@ export interface UseGitRepoResult {
 	/** Delete a branch. */
 	deleteBranch: (name: string, force?: boolean) => Promise<void>;
 	/** Merge source into current (or target) branch. */
-	merge: (sourceBranch: string, targetBranch?: string, message?: string) => Promise<void>;
+	merge: (
+		sourceBranch: string,
+		targetBranch?: string,
+		message?: string,
+	) => Promise<void>;
 	/** Clear the current error. */
 	clearError: () => void;
 }
@@ -90,7 +94,7 @@ export function useGitRepo(
 	const [error, setError] = useState<string | null>(null);
 
 	const withLoading = useCallback(
-		async <T,>(fn: () => Promise<T>): Promise<T> => {
+		async <T>(fn: () => Promise<T>): Promise<T> => {
 			setLoading(true);
 			setError(null);
 			try {
@@ -157,9 +161,7 @@ export function useGitRepo(
 	const commit = useCallback(
 		async (message: string, files?: string[]) => {
 			if (!projectId || !repoId) return;
-			await withLoading(() =>
-				commitChanges(projectId, repoId, message, files),
-			);
+			await withLoading(() => commitChanges(projectId, repoId, message, files));
 			await refreshStatus();
 		},
 		[projectId, repoId, withLoading, refreshStatus],