feat(attachments): data layer for task file attachments

TaskAttachmentEntity (+config, cascade FK), TaskAttachmentRepository, and an
AttachmentStore that writes files under ~/.todo-app/attachments/<taskId>/ with
a path-traversal guard and a 5 MB cap. TaskPromptComposer gains an optional
read-only 'Reference files' section. Migration AddTaskAttachments.

src/ClaudeDo.Data/ClaudeDoDbContext.cs

@@ -46,6 +46,7 @@ public class ClaudeDoDbContext : DbContext
     public DbSet<WorktreeEntity> Worktrees => Set<WorktreeEntity>();
     public DbSet<TaskRunEntity> TaskRuns => Set<TaskRunEntity>();
     public DbSet<SubtaskEntity> Subtasks => Set<SubtaskEntity>();
+    public DbSet<TaskAttachmentEntity> TaskAttachments => Set<TaskAttachmentEntity>();
     public DbSet<AppSettingsEntity> AppSettings => Set<AppSettingsEntity>();
     public DbSet<PrimeScheduleEntity> PrimeSchedules => Set<PrimeScheduleEntity>();
     public DbSet<DailyNoteEntity> DailyNotes => Set<DailyNoteEntity>();