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.Worker/External/ExternalMcpService.cs

@@ -39,7 +39,7 @@ public sealed record TaskDto(
 
 public sealed record WorktreeInfoDto(
     string Path, string Branch, string HeadCommit, string BaseCommit,
-    int Ahead, int Behind, bool IsDirty);
+    int Ahead, int Behind, bool IsDirty, string? MergeCommit = null);
 
 public sealed record TaskDiffDto(
     string Content, IReadOnlyList<string> Files, bool Truncated, int TotalBytes);
@@ -59,6 +59,9 @@ public sealed record WorktreeListItemDto(
 public sealed record CleanupWorktreeResult(
     bool Removed, string WorktreePath, bool BranchDeleted);
 
+public sealed record RevertMergeResultDto(
+    bool Reverted, string? RevertCommit, IReadOnlyList<string> Conflicts, string? Message);
+
 public sealed record DailyPrepCandidateDto(
     string Id, string ListId, string ListName, string Title, string? Description,
     bool IsStarred, DateTime? ScheduledFor, DateTime CreatedAt);
@@ -486,7 +489,10 @@ public sealed class ExternalMcpService
         "Get git worktree details for a task: path, branch, headCommit (current HEAD SHA), " +
         "baseCommit (SHA where the branch was created), ahead (commits on branch since base), " +
         "behind (commits on main not yet on this branch; 0 if 'main' ref is unreachable), " +
-        "isDirty (has uncommitted changes in the worktree directory). " +
+        "isDirty (has uncommitted changes in the worktree directory), " +
+        "mergeCommit (SHA of the merge commit this worktree's branch produced on the target branch, " +
+        "if it has been merged and that succeeded after this field was introduced; null otherwise — " +
+        "required by revert_merge). " +
         "Throws if the task or its worktree does not exist.")]
     public async Task<WorktreeInfoDto> GetTaskWorktree(string taskId, CancellationToken cancellationToken)
     {
@@ -500,7 +506,7 @@ public sealed class ExternalMcpService
         var ahead  = await GitRevListCountAsync(wt.Path, $"{wt.BaseCommit}..HEAD", cancellationToken);
         var behind = await GitRevListCountAsync(wt.Path, "HEAD..main", cancellationToken);
 
-        return new WorktreeInfoDto(wt.Path, wt.BranchName, headCommit!, wt.BaseCommit, ahead, behind, isDirty);
+        return new WorktreeInfoDto(wt.Path, wt.BranchName, headCommit!, wt.BaseCommit, ahead, behind, isDirty, wt.MergeCommit);
     }
 
     [McpServerTool, Description(
@@ -729,6 +735,34 @@ public sealed class ExternalMcpService
         return ToDto((await _tasks.GetByIdAsync(taskId, cancellationToken))!);
     }
 
+    [McpServerTool, Description(
+        "Revert a previously merged task's merge commit on targetBranch (default: main), via `git revert -m 1` — " +
+        "a new commit, never a reset/rewrite (the target working directory is shared with other sessions). " +
+        "Requires the task to be Done with a Merged worktree that has a recorded merge commit; tasks merged " +
+        "before this feature existed have no recorded commit and are refused rather than guessed via git log. " +
+        "On success: reverted=true, revertCommit is the new commit's SHA, and the task returns to " +
+        "WaitingForReview so it can be reconsidered. " +
+        "On a conflicting revert: reverted=false, the revert is aborted immediately (no half-resolved state " +
+        "left in the tree) and conflicts lists the files that would have conflicted. " +
+        "Throws if there is no recorded merge commit, the repo is mid-merge/mid-revert, or the target working " +
+        "tree has uncommitted changes from another session.")]
+    public async Task<RevertMergeResultDto> RevertMerge(
+        string taskId, string targetBranch = "main", CancellationToken cancellationToken = default)
+    {
+        var result = await _merge.RevertMergeAsync(taskId, targetBranch, cancellationToken);
+
+        if (result.Status == TaskMergeService.StatusReverted)
+        {
+            await _broadcaster.TaskUpdated(taskId);
+            return new RevertMergeResultDto(true, result.RevertCommit, Array.Empty<string>(), null);
+        }
+
+        if (result.Status == TaskMergeService.StatusConflictAborted)
+            return new RevertMergeResultDto(false, null, result.ConflictFiles, result.ErrorMessage);
+
+        throw new InvalidOperationException(result.ErrorMessage ?? $"Revert blocked: {result.Status}");
+    }
+
     [McpServerTool, Description(
         "List all ClaudeDo-tracked worktrees. " +
         "Each entry: taskId, path, branch, headCommit (empty if path missing on disk), " +