fix(worker,data): honor wait_for_task_change's NotFound contract, harden TaskNumberAllocator

wait_for_task_change resolved #<number>/bare-number ids up front via TaskIdResolver, which
throws for an unknown number -- breaking the tool's own documented promise that an unknown
id reports status "NotFound" instead of failing the whole call. Resolve per id and fall back
to the original id on a resolution failure so CheckOnceAsync can still report it.

TaskNumberAllocator.AddWithNumberAsync indexed into the app_settings UPDATE...RETURNING result
without checking for an empty result, throwing on a missing singleton row; it also caught any
DbUpdateException as a number collision, burning up to 5 numbers on an unrelated failure (e.g.
FK violation) before the real error surfaced. Now recreates the missing row and only retries on
the actual unique-index collision (SQLite error 19 on tasks.number), rethrowing everything else
immediately.

Audited the other TaskIdResolver.ResolveAsync/ResolveManyAsync call sites (ExternalMcpService,
HandoffMcpTools, ConfigMcpTools, RunHistoryMcpTools, AttachmentMcpTools, LifecycleMcpTools,
BatchMcpTools): none of their tool descriptions promise a found/NotFound flag for the id itself
(BatchMcpTools.BatchGetTasks already handles this correctly via its own per-id try/catch;
PreviewMergeSet promises a per-task "error" field, not a found/NotFound flag; the rest are
single-id tools that already throw on a missing task downstream) -- left throwing behavior as-is.

src/ClaudeDo.Data/TaskNumberAllocator.cs

@@ -1,4 +1,5 @@
 using ClaudeDo.Data.Models;
+using Microsoft.Data.Sqlite;
 using Microsoft.EntityFrameworkCore;
 
 namespace ClaudeDo.Data;
@@ -29,6 +30,16 @@ public static class TaskNumberAllocator
                 RETURNING *
                 """, AppSettingsEntity.SingletonId).AsNoTracking().ToListAsync(ct);
 
+            if (settings.Count == 0)
+            {
+                // The singleton row is missing (e.g. a corrupted/hand-edited DB) -- recreate it,
+                // as its own separate insert/commit, and retry the UPDATE above rather than
+                // indexing into an empty result. Doesn't count against MaxAttempts: it's not a
+                // number collision.
+                await EnsureSingletonRowAsync(context, ct);
+                continue;
+            }
+
             entity.Number = settings[0].NextTaskNumber - 1;
             context.Tasks.Add(entity);
 
@@ -37,10 +48,40 @@ public static class TaskNumberAllocator
                 await context.SaveChangesAsync(ct);
                 return;
             }
-            catch (DbUpdateException) when (attempt < MaxAttempts)
+            catch (DbUpdateException ex)
             {
                 context.Entry(entity).State = EntityState.Detached;
+                if (attempt < MaxAttempts && IsNumberCollision(ex))
+                    continue;
+                throw;
             }
         }
     }
+
+    private static async Task EnsureSingletonRowAsync(ClaudeDoDbContext context, CancellationToken ct)
+    {
+        var row = new AppSettingsEntity { Id = AppSettingsEntity.SingletonId };
+        context.AppSettings.Add(row);
+        try
+        {
+            await context.SaveChangesAsync(ct);
+        }
+        catch (DbUpdateException)
+        {
+            // A concurrent process already inserted the singleton -- fine, the next UPDATE
+            // attempt above will find it.
+        }
+        finally
+        {
+            context.Entry(row).State = EntityState.Detached;
+        }
+    }
+
+    // Distinguishes the expected unique-index collision on idx_tasks_number (worth retrying with
+    // a fresh number) from any other DbUpdateException -- an FK violation, a NOT NULL violation,
+    // etc. -- which must surface immediately instead of silently burning up to MaxAttempts numbers
+    // on a failure a retry can never fix.
+    private static bool IsNumberCollision(DbUpdateException ex) =>
+        ex.InnerException is SqliteException { SqliteErrorCode: 19 } sqliteEx
+        && sqliteEx.Message.Contains("tasks.number", StringComparison.Ordinal);
 }

src/ClaudeDo.Worker/External/TaskWaitMcpTools.cs

@@ -109,7 +109,19 @@ public sealed class TaskWaitMcpTools
         var tasks = new TaskRepository(ctx);
         var resolved = new string[ids.Length];
         for (var i = 0; i < ids.Length; i++)
-            resolved[i] = await TaskIdResolver.ResolveAsync(tasks, ids[i], ct);
+        {
+            try
+            {
+                resolved[i] = await TaskIdResolver.ResolveAsync(tasks, ids[i], ct);
+            }
+            catch (InvalidOperationException)
+            {
+                // A #<number>/bare-number id with no matching task -- keep the original,
+                // unresolvable id so CheckOnceAsync's lookup misses and reports "NotFound",
+                // per this tool's documented contract, instead of failing the whole call.
+                resolved[i] = ids[i];
+            }
+        }
         return resolved;
     }