feat(attachments): inject reference files into the run + clean up files on delete

TaskRunner appends attached files (absolute paths) to the run prompt as the
read-only Reference files section. Task and list deletes now remove the
on-disk attachment dir eagerly, and a startup AttachmentOrphanRecovery sweep
drops any attachments/<taskId>/ whose task no longer exists (covers list
cascade and planning-discard paths).

src/ClaudeDo.Worker/Lifecycle/AttachmentOrphanRecovery.cs

@@ -0,0 +1,51 @@
+using ClaudeDo.Data;
+using Microsoft.EntityFrameworkCore;
+
+namespace ClaudeDo.Worker.Lifecycle;
+
+/// <summary>
+/// Startup-only sweep: deletes attachment directories whose task no longer exists in the DB.
+/// </summary>
+public sealed class AttachmentOrphanRecovery : IHostedService
+{
+    private readonly IDbContextFactory<ClaudeDoDbContext> _dbFactory;
+    private readonly AttachmentStore _store;
+    private readonly ILogger<AttachmentOrphanRecovery> _logger;
+
+    public AttachmentOrphanRecovery(
+        IDbContextFactory<ClaudeDoDbContext> dbFactory,
+        AttachmentStore store,
+        ILogger<AttachmentOrphanRecovery> logger)
+    {
+        _dbFactory = dbFactory;
+        _store = store;
+        _logger = logger;
+    }
+
+    public async Task StartAsync(CancellationToken cancellationToken)
+    {
+        var taskIds = _store.EnumerateTaskIds();
+        if (taskIds.Count == 0)
+        {
+            _logger.LogInformation("Attachment orphan recovery: no attachment directories found");
+            return;
+        }
+
+        await using var ctx = await _dbFactory.CreateDbContextAsync(cancellationToken);
+        var existingIds = (await ctx.Tasks
+            .Where(t => taskIds.Contains(t.Id))
+            .Select(t => t.Id)
+            .ToListAsync(cancellationToken)).ToHashSet();
+
+        var orphans = taskIds.Where(id => !existingIds.Contains(id)).ToList();
+        foreach (var id in orphans)
+            _store.DeleteTaskDir(id);
+
+        if (orphans.Count > 0)
+            _logger.LogWarning("Attachment orphan recovery: removed {Count} orphaned attachment director(ies)", orphans.Count);
+        else
+            _logger.LogInformation("Attachment orphan recovery: no orphaned attachment directories found");
+    }
+
+    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+}