feat(data): add task numbers schema, allocator, and backfill migration

TaskEntity.Number is a global, monotonically increasing, never-reused
integer (displayed as #123), allocated from AppSettingsEntity.NextTaskNumber
via a single UPDATE...RETURNING statement rather than MAX(number)+1, which
would reissue a deleted task's number. Both insert paths (TaskRepository.
AddAsync and CreateChildAsync) route through the new TaskNumberAllocator,
with a bounded retry on a unique-index collision. One migration adds the
columns, backfills existing rows in creation order, and creates the unique
index afterwards. Data-layer only; MCP/UI wiring is later slices.

tests/ClaudeDo.Worker.Tests/Planning/PlanningMergeOrchestratorTests.cs

@@ -47,6 +47,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
     private readonly List<DbFixture> _dbs = new();
     private readonly List<GitRepoFixture> _repos = new();
     private readonly List<(string repoDir, string wtPath)> _wtCleanups = new();
+    private int _numberSeed;
 
     private DbFixture NewDb() { var d = new DbFixture(); _dbs.Add(d); return d; }
     private GitRepoFixture NewRepo() { var r = new GitRepoFixture(); _repos.Add(r); return r; }
@@ -102,6 +103,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
             Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized, SortOrder = 0,
+            Number = ++_numberSeed,
         });
 
         var subA = Guid.NewGuid().ToString();
@@ -110,11 +112,13 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = subA, ListId = listId, Title = "child A", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+            Number = ++_numberSeed,
         });
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subB, ListId = listId, Title = "child B", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+            Number = ++_numberSeed,
         });
         await ctx.SaveChangesAsync();
 
@@ -170,14 +174,15 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
             Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.Finalized, SortOrder = 0,
+            Number = ++_numberSeed,
         });
         var subA = Guid.NewGuid().ToString();
         var subB = Guid.NewGuid().ToString();
         var subC = Guid.NewGuid().ToString();
         ctx.Tasks.AddRange(
-            new TaskEntity { Id = subA, ListId = listId, Title = "A", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1 },
-            new TaskEntity { Id = subB, ListId = listId, Title = "B", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2 },
-            new TaskEntity { Id = subC, ListId = listId, Title = "C", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 3 }
+            new TaskEntity { Id = subA, ListId = listId, Title = "A", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1, Number = ++_numberSeed },
+            new TaskEntity { Id = subB, ListId = listId, Title = "B", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2, Number = ++_numberSeed },
+            new TaskEntity { Id = subC, ListId = listId, Title = "C", CreatedAt = DateTime.UtcNow, ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 3, Number = ++_numberSeed }
         );
         await ctx.SaveChangesAsync();
 
@@ -509,6 +514,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = parentId, ListId = listId, Title = "plan", CreatedAt = DateTime.UtcNow,
             Status = TaskStatus.Idle, PlanningPhase = PlanningPhase.Finalized, SortOrder = 0,
+            Number = ++_numberSeed,
         });
         var running = Guid.NewGuid().ToString();
         ctx.Tasks.Add(new TaskEntity
@@ -516,6 +522,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
             Id = running, ListId = listId, Title = "still running",
             CreatedAt = DateTime.UtcNow, ParentTaskId = parentId,
             Status = TaskStatus.Running, SortOrder = 1,
+            Number = ++_numberSeed,
         });
         await ctx.SaveChangesAsync();
         SeedWorktreeWithFile(ctx, repo, running, "fileR.txt", "R\n");
@@ -574,6 +581,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = parentId, ListId = listId, Title = "improve", CreatedAt = DateTime.UtcNow,
             Status = TaskStatus.WaitingForReview, PlanningPhase = PlanningPhase.None, SortOrder = 0,
+            Number = ++_numberSeed,
         });
 
         var subA = Guid.NewGuid().ToString();
@@ -582,11 +590,13 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = subA, ListId = listId, Title = "child A", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+            Number = ++_numberSeed,
         });
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subB, ListId = listId, Title = "child B", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+            Number = ++_numberSeed,
         });
         await ctx.SaveChangesAsync();
 
@@ -642,6 +652,7 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = parentId, ListId = listId, Title = "improve", CreatedAt = DateTime.UtcNow,
             Status = TaskStatus.Cancelled, PlanningPhase = PlanningPhase.None, SortOrder = 0,
+            Number = ++_numberSeed,
         });
 
         var subA = Guid.NewGuid().ToString();
@@ -650,11 +661,13 @@ public sealed class PlanningMergeOrchestratorTests : IDisposable
         {
             Id = subA, ListId = listId, Title = "child A", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 1,
+            Number = ++_numberSeed,
         });
         ctx.Tasks.Add(new TaskEntity
         {
             Id = subB, ListId = listId, Title = "child B", CreatedAt = DateTime.UtcNow,
             ParentTaskId = parentId, Status = TaskStatus.Done, SortOrder = 2,
+            Number = ++_numberSeed,
         });
         await ctx.SaveChangesAsync();