fix(worker): make re-approve after revert_merge do a real merge, not a silent no-op

ApproveAndMergeAsync treated any non-Active worktree (including Kept, the
state RevertMergeAsync leaves behind) the same as "never had a worktree",
so re-approving a reverted task ran only the verify gate and marked the
task Done without merging anything back.

Now a Kept worktree is distinguished: if its branch is gone (the common
case, since approve normally deletes it), approve returns Blocked instead
of a fake "merged". If the branch survives, its tip is already an ancestor
of the target (from the original merge), so a plain `git merge` would
silently no-op ("Already up to date."); cherry-picking the original merge
commit's own diff (-m 1) is what actually restores the content, recorded
as a fresh MergeCommit.

src/ClaudeDo.Data/Git/GitService.cs

@@ -356,6 +356,38 @@ public sealed class GitService
             throw new InvalidOperationException($"git revert --abort failed (exit {exitCode}): {stderr}");
     }
 
+    /// <summary>
+    /// Re-applies a merge commit's own diff (against its first parent, `-m 1`) as a new commit on
+    /// top of HEAD. Needed to redo a merge that was previously reverted: once a branch tip is an
+    /// ancestor of HEAD, `git merge` on that branch again is a silent "Already up to date." no-op
+    /// (ancestry, not content, is what merge checks) -- cherry-picking the original merge commit's
+    /// diff is what actually restores the content.
+    /// </summary>
+    public async Task<(int ExitCode, string Stderr)> CherryPickMergeCommitAsync(
+        string repoDir, string mergeCommitSha, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir,
+            ["-c", "merge.conflictStyle=diff3", "cherry-pick", "-m", "1", "--no-edit", mergeCommitSha], ct);
+        return (exitCode, stderr);
+    }
+
+    public async Task<bool> IsMidCherryPickAsync(string repoDir, CancellationToken ct = default)
+    {
+        var (exitCode, stdout, _) = await RunGitAsync(repoDir, ["rev-parse", "--git-dir"], ct);
+        if (exitCode != 0) return false;
+        var gitDir = stdout.Trim();
+        if (!Path.IsPathRooted(gitDir))
+            gitDir = Path.Combine(repoDir, gitDir);
+        return File.Exists(Path.Combine(gitDir, "CHERRY_PICK_HEAD"));
+    }
+
+    public async Task CherryPickAbortAsync(string repoDir, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["cherry-pick", "--abort"], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git cherry-pick --abort failed (exit {exitCode}): {stderr}");
+    }
+
     public async Task<List<string>> ListConflictedFilesAsync(string repoDir, CancellationToken ct = default)
     {
         var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,