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.

tests/ClaudeDo.Data.Tests/ForeignKeyTests.cs

@@ -69,6 +69,35 @@ public sealed class ForeignKeyTests : IDisposable
         }
     }
 
+    [Fact]
+    public async Task DependsOnTaskId_is_nulled_when_predecessor_deleted_on_fresh_context()
+    {
+        var listId = Guid.NewGuid().ToString();
+        var predecessorId = Guid.NewGuid().ToString();
+        var dependentId = Guid.NewGuid().ToString();
+
+        await using (var ctx = Open())
+        {
+            ctx.Lists.Add(new ListEntity { Id = listId, Name = "L", CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = predecessorId, ListId = listId, Title = "Predecessor", Status = TaskStatus.Idle, CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = dependentId, ListId = listId, Title = "Dependent", Status = TaskStatus.Idle, DependsOnTaskId = predecessorId, CreatedAt = DateTime.UtcNow });
+            await ctx.SaveChangesAsync();
+        }
+
+        await using (var ctx = Open())
+        {
+            var predecessor = await ctx.Tasks.FindAsync(predecessorId);
+            ctx.Tasks.Remove(predecessor!);
+            await ctx.SaveChangesAsync();
+        }
+
+        await using (var ctx = Open())
+        {
+            var dependent = await ctx.Tasks.AsNoTracking().FirstAsync(t => t.Id == dependentId);
+            Assert.Null(dependent.DependsOnTaskId);
+        }
+    }
+
     // ---- AppSettingsRepository: get-or-create resilience ----
 
     [Fact]