chore(claude-do): Fix inconsistent timezone on timestamps

Timestamps are serialized inconsistently across tools. add_task returns createdAt with a trailing 'Z' (e.g. "2026-06-01T13:03:56.1636946Z"), but get_task and list_runs return the same value WITHOUT the 'Z'. This is a timezone-ambiguity bug.

Fix: serialize all DateTime values as UTC with the 'Z' suffix consistently (use a single shared JSON serializer setting / DateTimeKind=Utc). Audit every tool

ClaudeDo-Task: 4bbc759e-ff05-45e3-a57f-b290c7e16264

src/ClaudeDo.Data/ClaudeDoDbContext.cs

@@ -3,6 +3,7 @@ using ClaudeDo.Data.Seeding;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore.Infrastructure;
 using Microsoft.EntityFrameworkCore.Storage;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
 
 namespace ClaudeDo.Data;
 
@@ -19,9 +20,24 @@ public class ClaudeDoDbContext : DbContext
     public DbSet<AppSettingsEntity> AppSettings => Set<AppSettingsEntity>();
     public DbSet<PrimeScheduleEntity> PrimeSchedules => Set<PrimeScheduleEntity>();
 
+    private static readonly ValueConverter<DateTime, DateTime> UtcConverter =
+        new(v => v, v => DateTime.SpecifyKind(v, DateTimeKind.Utc));
+
+    private static readonly ValueConverter<DateTime?, DateTime?> UtcNullableConverter =
+        new(v => v, v => v.HasValue ? DateTime.SpecifyKind(v.Value, DateTimeKind.Utc) : null);
+
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         modelBuilder.ApplyConfigurationsFromAssembly(typeof(ClaudeDoDbContext).Assembly);
+
+        foreach (var entityType in modelBuilder.Model.GetEntityTypes())
+        foreach (var property in entityType.GetProperties())
+        {
+            if (property.ClrType == typeof(DateTime) && property.GetValueConverter() == null)
+                property.SetValueConverter(UtcConverter);
+            else if (property.ClrType == typeof(DateTime?) && property.GetValueConverter() == null)
+                property.SetValueConverter(UtcNullableConverter);
+        }
     }
 
     /// <summary>