Merge subtask

# Conflicts:
#	src/ClaudeDo.Data/Git/GitService.cs
#	src/ClaudeDo.Worker/Lifecycle/TaskMergeService.cs

src/ClaudeDo.Data/Git/GitService.cs

@@ -493,6 +493,28 @@ public sealed class GitService
             .ToList();
     }
 
+    /// <summary>
+    /// Rebases the branch checked out at <paramref name="worktreePath"/> onto <paramref name="ontoRef"/>.
+    /// On conflict or any other failure the rebase is aborted before returning, so the worktree is left
+    /// exactly as it was rather than stranded mid-rebase; <c>ConflictFiles</c> is best-effort and only
+    /// populated when the failure was an actual conflict.
+    /// </summary>
+    public async Task<(int ExitCode, string Stderr, IReadOnlyList<string> ConflictFiles)> RebaseAsync(
+        string worktreePath, string ontoRef, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(worktreePath, ["rebase", ontoRef], ct);
+        if (exitCode == 0)
+            return (0, stderr, Array.Empty<string>());
+
+        List<string> conflictFiles;
+        try { conflictFiles = await ListConflictedFilesAsync(worktreePath, ct); }
+        catch { conflictFiles = new(); }
+
+        await RunGitAsync(worktreePath, ["rebase", "--abort"], ct);
+        return (exitCode, stderr, conflictFiles);
+    }
+
+
     private static async Task<(int ExitCode, string Stdout, string Stderr)> RunGitAsync(
         string workDir, IEnumerable<string> args, CancellationToken ct, string? stdinData = null, bool trimOutput = true)
     {