refactor(tags): remove tag entity and all references

Drops TagEntity, TagRepository, and tag wiring across data layer, worker,
and UI. Adds RemoveTags migration to clean up schema.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

@@ -65,7 +65,6 @@ public sealed class PlanningChainCoordinatorTests : IDisposable
         await using var ctx = _factory.CreateDbContext();
         return await ctx.Tasks
             .AsNoTracking()
-            .Include(t => t.Tags)
             .Where(t => t.ParentTaskId == parentId)
             .OrderBy(t => t.SortOrder)
             .ToListAsync();
@@ -88,17 +87,6 @@ public sealed class PlanningChainCoordinatorTests : IDisposable
         Assert.Equal(kids[1].Id, kids[2].BlockedByTaskId);
     }
 
-    [Fact]
-    public async Task SetupChain_AttachesAgentTagToAllChildren()
-    {
-        await SeedPlanningFamilyAsync("P", 2);
-
-        await _sut.SetupChainAsync("P", default);
-
-        var kids = await GetChildrenAsync("P");
-        Assert.All(kids, k => Assert.Contains(k.Tags, t => t.Name == "agent"));
-    }
-
     [Fact]
     public async Task SetupChain_AcceptsIdleChildren()
     {