feat(worker): auto-rebase overlapping WaitingForReview branches after a merge

After a merge lands on the target branch, TaskMergeService now rebases every
other WaitingForReview task's active worktree onto the new tip -- but only
when that branch actually touches a file the merge just changed, computed via
GitService.GetChangedFileNamesAsync (both the merge's landed files and each
candidate's own diff). A branch merely behind is left alone. On conflict or
any other failure the rebase is aborted and the branch is left exactly as it
was, with a WorkerLog warning naming the task; a successful rebase updates the
worktree's recorded BaseCommit/HeadCommit and broadcasts WorktreeUpdated.

src/ClaudeDo.Data/Git/GitService.cs

@@ -389,6 +389,42 @@ public sealed class GitService
             .Count(s => s.Length > 0);
     }
 
+    /// <summary>Files that differ between two exact refs (2-dot, no merge-base resolution) -- used to see
+    /// what a target branch itself picked up since a task's fork point, as opposed to
+    /// <see cref="CountChangedFilesAsync"/>'s 3-dot count of a branch's own changes.</summary>
+    public async Task<IReadOnlyList<string>> GetChangedFileNamesAsync(
+        string repoDir, string fromRef, string toRef, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir,
+            ["diff", "--name-only", $"{fromRef}..{toRef}"], ct);
+        if (exitCode != 0) return Array.Empty<string>();
+        return stdout
+            .Split('\n', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries)
+            .Where(s => s.Length > 0)
+            .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)
     {