feat(worker): record merge commit SHA and add revert_merge tool

Persists the merge commit SHA on WorktreeEntity for every successful
single-task and unit merge, and adds a TaskMergeService.RevertMergeAsync
+ revert_merge MCP tool that undoes a merged task's merge via
`git revert -m 1` (never reset/rewrite, since the target checkout is
shared). Rejects cleanly when there's no recorded SHA, the repo is
mid-merge/mid-revert, or the target has foreign uncommitted changes;
a conflicting revert aborts immediately. Also exposes the new
mergeCommit field via get_task_worktree.

src/ClaudeDo.Data/Git/GitService.cs

@@ -280,6 +280,36 @@ public sealed class GitService
             throw new InvalidOperationException($"git merge --abort failed (exit {exitCode}): {stderr}");
     }
 
+    public async Task<bool> IsMidRevertAsync(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, "REVERT_HEAD"));
+    }
+
+    /// <summary>
+    /// Reverts a single commit with `-m 1` (diff against its first parent) — the form needed to
+    /// revert a merge commit. On success this creates a new commit with the inverse changes;
+    /// the original commit and all history stay intact (no rewrite, no reset).
+    /// </summary>
+    public async Task<(int ExitCode, string Stderr)> RevertMergeCommitAsync(
+        string repoDir, string mergeCommitSha, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir,
+            ["-c", "merge.conflictStyle=diff3", "revert", "--no-edit", "-m", "1", mergeCommitSha], ct);
+        return (exitCode, stderr);
+    }
+
+    public async Task RevertAbortAsync(string repoDir, CancellationToken ct = default)
+    {
+        var (exitCode, _, stderr) = await RunGitAsync(repoDir, ["revert", "--abort"], ct);
+        if (exitCode != 0)
+            throw new InvalidOperationException($"git revert --abort failed (exit {exitCode}): {stderr}");
+    }
+
     public async Task<List<string>> ListConflictedFilesAsync(string repoDir, CancellationToken ct = default)
     {
         var (exitCode, stdout, stderr) = await RunGitAsync(repoDir,