feat: workspace creation with branch dropdown and auto-select

- Fetch branches from selected repo via listRepositoryBranches API
- Branch dropdown with default branch pre-selected
- '+ Create new branch...' option reveals text input for custom branch
- Auto-select first option when only one available:
  - Project: auto-selects when only 1 project
  - Repo: auto-selects when only 1 repo
  - Branch: auto-selects when only 1 branch, otherwise defaults to remote default
- Falls back to free-text branch input if branch API fails
- TypeScript + eslint clean

apps/web/src/pages/workspaces.tsx

@@ -8,7 +8,7 @@ import { WorkspaceCard } from "../components/workspace-card";
 import { StartToolModal } from "../components/start-tool-modal";
 import { createInstance, startInstance } from "../api/sessions";
 import { listProjects } from "../api/projects";
-import { listRepositories } from "../api/git_repositories";
+import { listRepositories, listRepositoryBranches } from "../api/git_repositories";
 import { createWorkspaceTopLevel } from "../api/workspaces";
 import type { Workspace } from "../types/workspace";
 import type { ProjectWithRepos } from "../types";
@@ -152,10 +152,13 @@ function WorkspaceCreateInline({
 }) {
 	const [projects, setProjects] = useState<ProjectWithRepos[]>([]);
 	const [repos, setRepos] = useState<GitRepository[]>([]);
+	const [branches, setBranches] = useState<string[]>([]);
 	const [selectedProject, setSelectedProject] = useState("");
 	const [selectedRepo, setSelectedRepo] = useState("");
+	const [selectedBranch, setSelectedBranch] = useState("");
+	const [newBranchName, setNewBranchName] = useState("");
+	const [isNewBranch, setIsNewBranch] = useState(false);
 	const [name, setName] = useState("");
-	const [branch, setBranch] = useState("main");
 	const [loading, setLoading] = useState(false);
 	const [fetching, setFetching] = useState(true);
 	const [error, setError] = useState<string | null>(null);
@@ -198,6 +201,45 @@ function WorkspaceCreateInline({
 		void loadRepos();
 	}, [selectedProject]);
 
+	useEffect(() => {
+		if (!selectedProject || !selectedRepo) {
+			setBranches([]);
+			setSelectedBranch("");
+			setIsNewBranch(false);
+			return;
+		}
+		const loadBranches = async () => {
+			try {
+				const data = await listRepositoryBranches(selectedProject, selectedRepo);
+				const branchNames = data.branches.map((b) => b.name);
+				setBranches(branchNames);
+				if (branchNames.length === 1) {
+					setSelectedBranch(branchNames[0]);
+					setIsNewBranch(false);
+				} else if (data.default_branch) {
+					setSelectedBranch(data.default_branch);
+					setIsNewBranch(false);
+				}
+			} catch {
+				// If branch fetch fails, fall back to free-text
+				setBranches([]);
+				setIsNewBranch(true);
+			}
+		};
+		void loadBranches();
+	}, [selectedProject, selectedRepo]);
+
+	const handleBranchChange = (value: string) => {
+		if (value === "__new__") {
+			setIsNewBranch(true);
+			setSelectedBranch("__new__");
+			setNewBranchName("");
+		} else {
+			setIsNewBranch(false);
+			setSelectedBranch(value);
+		}
+	};
+
 	const handleSubmit = async (e: React.FormEvent) => {
 		e.preventDefault();
 		if (!selectedRepo) {
@@ -208,17 +250,24 @@ function WorkspaceCreateInline({
 			setError("Workspace name is required");
 			return;
 		}
+		const branchName = isNewBranch ? newBranchName.trim() : selectedBranch;
+		if (!branchName) {
+			setError("Please select or enter a branch");
+			return;
+		}
 		setLoading(true);
 		setError(null);
 		try {
 			await createWorkspaceTopLevel({
 				repo_id: selectedRepo,
 				name: name.trim(),
-				branch: branch.trim() || "main",
+				branch: branchName,
 			});
 			onCreated();
 		} catch (err) {
-			setError(err instanceof Error ? err.message : "Failed to create workspace");
+			setError(
+				err instanceof Error ? err.message : "Failed to create workspace",
+			);
 		} finally {
 			setLoading(false);
 		}
@@ -292,12 +341,47 @@ function WorkspaceCreateInline({
 					<label>
 						<Icon name="branch" size="sm" /> Branch
 					</label>
-					<input
-						type="text"
-						value={branch}
-						onChange={(e) => setBranch(e.target.value)}
-						placeholder="main"
-					/>
+					{branches.length > 0 ? (
+						<>
+							<select
+								value={selectedBranch}
+								onChange={(e) => handleBranchChange(e.target.value)}
+								required
+								disabled={!selectedRepo}
+							>
+								<option value="">Select branch...</option>
+								{branches.map((b) => (
+									<option key={b} value={b}>
+										{b}
+									</option>
+								))}
+								<option value="__new__">+ Create new branch...</option>
+							</select>
+							{isNewBranch && (
+								<input
+									type="text"
+									value={newBranchName}
+									onChange={(e) => setNewBranchName(e.target.value)}
+									placeholder="new-branch-name"
+									required
+									style={{ marginTop: "0.5rem" }}
+								/>
+							)}
+						</>
+					) : (
+						<input
+							type="text"
+							value={isNewBranch ? newBranchName : selectedBranch}
+							onChange={(e) => {
+								setIsNewBranch(true);
+								setNewBranchName(e.target.value);
+								setSelectedBranch("__new__");
+							}}
+							placeholder="main"
+							required
+							disabled={!selectedRepo}
+						/>
+					)}
 				</div>
 
 				{error && (