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>

src/ClaudeDo.Worker/Planning/PlanningChainCoordinator.cs

@@ -28,7 +28,6 @@ public sealed class PlanningChainCoordinator
     //   chain leaves history alone but still reshapes the tail.
     // - Running children abort the operation — the chain cannot be reshaped while
     //   one of its members is mid-flight.
-    // The "agent" tag is auto-attached to every child so the picker can claim them.
     // Returns the number of children placed in the chain.
     internal async Task<int> SetupChainAsync(string parentTaskId, CancellationToken ct = default)
     {
@@ -37,7 +36,6 @@ public sealed class PlanningChainCoordinator
             ?? throw new InvalidOperationException($"Task {parentTaskId} not found.");
 
         var children = await ctx.Tasks
-            .Include(t => t.Tags)
             .Where(t => t.ParentTaskId == parentTaskId)
             .OrderBy(t => t.SortOrder).ThenBy(t => t.CreatedAt)
             .ToListAsync(ct);
@@ -49,18 +47,6 @@ public sealed class PlanningChainCoordinator
             throw new InvalidOperationException(
                 $"Child {running.Id} is running; cannot reshape chain.");
 
-        // Worker queue picker requires the "agent" tag — attach it so children are pickable.
-        var agentTag = await ctx.Tags.FirstOrDefaultAsync(t => t.Name == "agent", ct);
-        if (agentTag is not null)
-        {
-            foreach (var c in children)
-            {
-                if (!c.Tags.Any(t => t.Id == agentTag.Id))
-                    c.Tags.Add(agentTag);
-            }
-            await ctx.SaveChangesAsync(ct);
-        }
-
         // Re-shape over Idle and Queued children only; leave Done/Failed/Cancelled
         // (terminal) results in place.
         var sequenceable = children