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/Configuration/WorktreeEntityConfiguration.cs

@@ -35,6 +35,7 @@ public class WorktreeEntityConfiguration : IEntityTypeConfiguration<WorktreeEnti
         builder.Property(w => w.BaseCommit).HasColumnName("base_commit").IsRequired();
         builder.Property(w => w.HeadCommit).HasColumnName("head_commit");
         builder.Property(w => w.DiffStat).HasColumnName("diff_stat");
+        builder.Property(w => w.MergeCommit).HasColumnName("merge_commit");
         builder.Property(w => w.State).HasColumnName("state").IsRequired()
             .HasDefaultValue(WorktreeState.Active)
             .HasConversion(StateConverter);