feat: add branch dropdown and new branch creation UI

- Replace free-text branch input with dropdown of available branches
- Add 'Create new branch...' option with name and base branch inputs
- Load branches from API when repository is selected in clone mode
- Pass newBranch parameter to createInstance API

apps/web/src/pages/sessions.tsx

@@ -3,7 +3,7 @@ import { useNavigate } from "react-router-dom";
 
 import { listProjects } from "../api/projects";
 import type { Project } from "../types";
-import { listRepositories, type GitRepository } from "../api/git_repositories";
+import { listRepositories, listRepositoryBranches, type GitRepository, type Branch } from "../api/git_repositories";
 import {
   getUserSessions,
   type Session,
@@ -43,6 +43,12 @@ export const SessionsPage = () => {
   const [branch, setBranch] = useState("main");
   const [sshKeys, setSshKeys] = useState<SSHKey[]>([]);
 
+  const [branches, setBranches] = useState<Branch[]>([]);
+  const [isLoadingBranches, setIsLoadingBranches] = useState(false);
+  const [isCreatingNewBranch, setIsCreatingNewBranch] = useState(false);
+  const [newBranchName, setNewBranchName] = useState("");
+  const [baseBranch, setBaseBranch] = useState("");
+
   const [dirtyDeleteSession, setDirtyDeleteSession] = useState<Session | null>(null);
   const [dirtyDeleteFiles, setDirtyDeleteFiles] = useState<string[]>([]);
 
@@ -116,6 +122,33 @@ export const SessionsPage = () => {
     void loadSshKeys();
   }, []);
 
+  useEffect(() => {
+    const loadBranches = async () => {
+      if (!selectedRepo || !selectedProject || cloneMode !== "clone") {
+        setBranches([]);
+        setIsCreatingNewBranch(false);
+        setNewBranchName("");
+        setBaseBranch("");
+        return;
+      }
+      setIsLoadingBranches(true);
+      try {
+        const data = await listRepositoryBranches(selectedProject, selectedRepo);
+        setBranches(data.branches);
+        const defaultBranch = data.default_branch;
+        setBaseBranch(defaultBranch);
+        if (!branch || !data.branches.find((b) => b.name === branch)) {
+          setBranch(defaultBranch);
+        }
+      } catch {
+        setBranches([]);
+      } finally {
+        setIsLoadingBranches(false);
+      }
+    };
+    void loadBranches();
+  }, [selectedRepo, selectedProject, cloneMode]);
+
   // Poll health every 30 seconds for active instances
   useEffect(() => {
     const checkHealth = async () => {
@@ -213,7 +246,8 @@ export const SessionsPage = () => {
         selectedToolType,
         displayName || undefined,
         cloneMode,
-        cloneMode === "clone" ? branch : undefined
+        isCreatingNewBranch ? baseBranch : branch,
+        isCreatingNewBranch ? newBranchName : undefined
       );
       
       // Auto-start the instance
@@ -227,6 +261,10 @@ export const SessionsPage = () => {
       setDisplayName("");
       setCloneMode("mount");
       setBranch("main");
+      setIsCreatingNewBranch(false);
+      setNewBranchName("");
+      setBaseBranch("");
+      setBranches([]);
       await loadSessions();
     } catch (error) {
       setCreateStatus("error");
@@ -687,14 +725,61 @@ export const SessionsPage = () => {
                   <>
                     <label className="form-field">
                       Branch
-                      <input
-                        type="text"
-                        value={branch}
-                        onChange={(e) => setBranch(e.target.value)}
-                        placeholder="main"
-                      />
+                      {isLoadingBranches ? (
+                        <span className="muted">Loading branches...</span>
+                      ) : (
+                        <select
+                          value={isCreatingNewBranch ? "__new__" : branch}
+                          onChange={(e) => {
+                            const value = e.target.value;
+                            if (value === "__new__") {
+                              setIsCreatingNewBranch(true);
+                              setNewBranchName("");
+                            } else {
+                              setIsCreatingNewBranch(false);
+                              setBranch(value);
+                              setBaseBranch(value);
+                            }
+                          }}
+                        >
+                          {branches.map((b) => (
+                            <option key={b.name} value={b.name}>
+                              {b.name} {b.is_default ? "(default)" : ""}
+                            </option>
+                          ))}
+                          <option value="__new__">Create new branch...</option>
+                        </select>
+                      )}
                     </label>
 
+                    {isCreatingNewBranch && (
+                      <>
+                        <label className="form-field">
+                          New Branch Name
+                          <input
+                            type="text"
+                            value={newBranchName}
+                            onChange={(e) => setNewBranchName(e.target.value)}
+                            placeholder="feature/my-new-branch"
+                            required
+                          />
+                        </label>
+                        <label className="form-field">
+                          Base Branch
+                          <select
+                            value={baseBranch}
+                            onChange={(e) => setBaseBranch(e.target.value)}
+                          >
+                            {branches.map((b) => (
+                              <option key={b.name} value={b.name}>
+                                {b.name} {b.is_default ? "(default)" : ""}
+                              </option>
+                            ))}
+                          </select>
+                        </label>
+                      </>
+                    )}
+
                     {selectedRepo && (
                       <div className="form-field ssh-key-info">
                         {(() => {