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/Repositories/WorktreeRepository.cs

@@ -36,6 +36,15 @@ public sealed class WorktreeRepository
             .ExecuteUpdateAsync(s => s.SetProperty(w => w.State, state), ct);
     }
 
+    public async Task SetMergedAsync(string taskId, string mergeCommit, CancellationToken ct = default)
+    {
+        await _context.Worktrees
+            .Where(w => w.TaskId == taskId)
+            .ExecuteUpdateAsync(s => s
+                .SetProperty(w => w.State, WorktreeState.Merged)
+                .SetProperty(w => w.MergeCommit, mergeCommit), ct);
+    }
+
     public async Task DeleteAsync(string taskId, CancellationToken ct = default)
     {
         await _context.Worktrees.Where(w => w.TaskId == taskId).ExecuteDeleteAsync(ct);