feat(worker): dependsOn via MCP + honest staleness signal in merge preview

Adds a user/MCP-declared task dependency (DependsOnTaskId) distinct from the
planning chain's internal BlockedByTaskId: add_task/update_task can set it,
the queue picker skips a Queued task until the dependency reaches Done, a
Failed/Cancelled dependency leaves the dependent blocked instead of starving
silently, and setting a link rejects self-reference/unknown-id/cycles.
get_task/list_tasks/batch_get_tasks now report blocked/blockedReason, and
wait_for_task_change reports "Blocked" immediately instead of running out its
timeout on a task the picker will never claim.

preview_merge/preview_merge_set gain staleFiles: files a branch touches that
the target branch also changed since the branch's fork point, a more honest
staleness signal than `behind` alone.

src/ClaudeDo.Data/Configuration/TaskEntityConfiguration.cs

@@ -74,6 +74,7 @@ public class TaskEntityConfiguration : IEntityTypeConfiguration<TaskEntity>
         builder.Property(t => t.PlanningPhase).HasColumnName("planning_phase").IsRequired()
             .HasConversion(PhaseConverter).HasDefaultValue(PlanningPhase.None);
         builder.Property(t => t.BlockedByTaskId).HasColumnName("blocked_by_task_id");
+        builder.Property(t => t.DependsOnTaskId).HasColumnName("depends_on_task_id");
         builder.Property(t => t.ScheduledFor).HasColumnName("scheduled_for");
         builder.Property(t => t.Result).HasColumnName("result");
         builder.Property(t => t.ReviewFeedback).HasColumnName("review_feedback");
@@ -115,6 +116,13 @@ public class TaskEntityConfiguration : IEntityTypeConfiguration<TaskEntity>
             .HasForeignKey(t => t.BlockedByTaskId)
             .OnDelete(DeleteBehavior.SetNull);
 
+        // DependsOn: user/MCP-declared predecessor. SetNull on delete so the dependent becomes
+        // pickable rather than blocked forever on a task that no longer exists.
+        builder.HasOne<TaskEntity>()
+            .WithMany()
+            .HasForeignKey(t => t.DependsOnTaskId)
+            .OnDelete(DeleteBehavior.SetNull);
+
         builder.HasOne(t => t.List)
             .WithMany(l => l.Tasks)
             .HasForeignKey(t => t.ListId)
@@ -129,5 +137,6 @@ public class TaskEntityConfiguration : IEntityTypeConfiguration<TaskEntity>
         builder.HasIndex(t => new { t.ListId, t.SortOrder }).HasDatabaseName("idx_tasks_list_sort");
         builder.HasIndex(t => t.ParentTaskId).HasDatabaseName("idx_tasks_parent_task_id");
         builder.HasIndex(t => t.BlockedByTaskId).HasDatabaseName("idx_tasks_blocked_by");
+        builder.HasIndex(t => t.DependsOnTaskId).HasDatabaseName("idx_tasks_depends_on");
     }
 }