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/components/workspace-create-form.tsx

@@ -57,8 +57,12 @@ export function WorkspaceCreateForm({
 					: git.branches[0];
 			setSelectedBranch(preferred);
 			setIsNewBranch(false);
+		} else if (git.error && git.branches.length === 0 && !isNewBranch) {
+			// API failed — default to manual entry so user can type a branch
+			setIsNewBranch(true);
+			setSelectedBranch("__manual__");
 		}
-	}, [git.branches, git.defaultBranch, selectedBranch]);
+	}, [git.branches, git.defaultBranch, git.error, selectedBranch, isNewBranch]);
 
 	/* ── Load projects (standalone mode only) ── */
 	const loadProjects = useCallback(async () => {
@@ -236,6 +240,20 @@ export function WorkspaceCreateForm({
 					<label>
 						<Icon name="branch" size="sm" /> Branch
 					</label>
+
+					{/* Show a hint when branches couldn’t be loaded */}
+					{git.error && git.branches.length === 0 && selectedRepo && (
+						<p
+							className="muted"
+							style={{
+								fontSize: "var(--font-size-xs)",
+								marginBottom: "0.25rem",
+							}}
+						>
+							Couldn’t load branches — type one manually.
+						</p>
+					)}
+
 					<select
 						value={selectedBranch}
 						onChange={(e) => handleBranchChange(e.target.value)}
@@ -258,14 +276,9 @@ export function WorkspaceCreateForm({
 						))}
 
 						<option value="__new__">+ Create new branch...</option>
-
-						{/* If branch fetch failed, offer manual entry */}
-						{git.error && git.branches.length === 0 && (
-							<option value="__manual__">✎ Enter branch name manually...</option>
-						)}
 					</select>
 
-					{/* New branch text input */}
+					{/* Text input for new branch or manual entry */}
 					{isNewBranch && (
 						<input
 							type="text"