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/Migrations/ClaudeDoDbContextModelSnapshot.cs

@@ -467,6 +467,10 @@ namespace ClaudeDo.Data.Migrations
                         .HasColumnType("TEXT")
                         .HasColumnName("created_by");
 
+                    b.Property<string>("DependsOnTaskId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("depends_on_task_id");
+
                     b.Property<string>("Description")
                         .HasColumnType("TEXT")
                         .HasColumnName("description");
@@ -600,6 +604,9 @@ namespace ClaudeDo.Data.Migrations
                     b.HasIndex("BlockedByTaskId")
                         .HasDatabaseName("idx_tasks_blocked_by");
 
+                    b.HasIndex("DependsOnTaskId")
+                        .HasDatabaseName("idx_tasks_depends_on");
+
                     b.HasIndex("ListId")
                         .HasDatabaseName("idx_tasks_list_id");
 
@@ -825,6 +832,11 @@ namespace ClaudeDo.Data.Migrations
                         .HasForeignKey("BlockedByTaskId")
                         .OnDelete(DeleteBehavior.SetNull);
 
+                    b.HasOne("ClaudeDo.Data.Models.TaskEntity", null)
+                        .WithMany()
+                        .HasForeignKey("DependsOnTaskId")
+                        .OnDelete(DeleteBehavior.SetNull);
+
                     b.HasOne("ClaudeDo.Data.Models.ListEntity", "List")
                         .WithMany("Tasks")
                         .HasForeignKey("ListId")