feat(data): add subtasks table, repository and prompt integration

Per-task checklist backend: subtasks table with CASCADE delete,
SubtaskEntity + SubtaskRepository (connection-per-op, async), DI
registration in App and Worker, TaskRunner composes a '## Sub-Tasks'
markdown block into the Claude prompt when subtasks exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Data/Models/SubtaskEntity.cs

@@ -0,0 +1,11 @@
+namespace ClaudeDo.Data.Models;
+
+public sealed class SubtaskEntity
+{
+    public required string Id { get; init; }
+    public required string TaskId { get; init; }
+    public required string Title { get; set; }
+    public bool Completed { get; set; }
+    public int OrderNum { get; set; }
+    public required DateTime CreatedAt { get; init; }
+}

src/ClaudeDo.Data/Repositories/SubtaskRepository.cs

@@ -0,0 +1,81 @@
+using ClaudeDo.Data.Models;
+using Microsoft.Data.Sqlite;
+
+namespace ClaudeDo.Data.Repositories;
+
+public sealed class SubtaskRepository
+{
+    private readonly SqliteConnectionFactory _factory;
+
+    public SubtaskRepository(SqliteConnectionFactory factory) => _factory = factory;
+
+    public async Task<List<SubtaskEntity>> GetByTaskIdAsync(string taskId, CancellationToken ct = default)
+    {
+        await using var conn = _factory.Open();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = "SELECT id, task_id, title, completed, order_num, created_at FROM subtasks WHERE task_id = @task_id ORDER BY order_num";
+        cmd.Parameters.AddWithValue("@task_id", taskId);
+
+        await using var reader = await cmd.ExecuteReaderAsync(ct);
+        var result = new List<SubtaskEntity>();
+        while (await reader.ReadAsync(ct))
+            result.Add(ReadSubtask(reader));
+        return result;
+    }
+
+    public async Task AddAsync(SubtaskEntity entity, CancellationToken ct = default)
+    {
+        await using var conn = _factory.Open();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = """
+            INSERT INTO subtasks (id, task_id, title, completed, order_num, created_at)
+            VALUES (@id, @task_id, @title, @completed, @order_num, @created_at)
+            """;
+        BindSubtask(cmd, entity);
+        await cmd.ExecuteNonQueryAsync(ct);
+    }
+
+    public async Task UpdateAsync(SubtaskEntity entity, CancellationToken ct = default)
+    {
+        await using var conn = _factory.Open();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = """
+            UPDATE subtasks SET title = @title, completed = @completed, order_num = @order_num
+            WHERE id = @id
+            """;
+        cmd.Parameters.AddWithValue("@id", entity.Id);
+        cmd.Parameters.AddWithValue("@title", entity.Title);
+        cmd.Parameters.AddWithValue("@completed", entity.Completed ? 1 : 0);
+        cmd.Parameters.AddWithValue("@order_num", entity.OrderNum);
+        await cmd.ExecuteNonQueryAsync(ct);
+    }
+
+    public async Task DeleteAsync(string id, CancellationToken ct = default)
+    {
+        await using var conn = _factory.Open();
+        await using var cmd = conn.CreateCommand();
+        cmd.CommandText = "DELETE FROM subtasks WHERE id = @id";
+        cmd.Parameters.AddWithValue("@id", id);
+        await cmd.ExecuteNonQueryAsync(ct);
+    }
+
+    private static void BindSubtask(SqliteCommand cmd, SubtaskEntity e)
+    {
+        cmd.Parameters.AddWithValue("@id", e.Id);
+        cmd.Parameters.AddWithValue("@task_id", e.TaskId);
+        cmd.Parameters.AddWithValue("@title", e.Title);
+        cmd.Parameters.AddWithValue("@completed", e.Completed ? 1 : 0);
+        cmd.Parameters.AddWithValue("@order_num", e.OrderNum);
+        cmd.Parameters.AddWithValue("@created_at", e.CreatedAt.ToString("o"));
+    }
+
+    private static SubtaskEntity ReadSubtask(SqliteDataReader r) => new()
+    {
+        Id = r.GetString(0),
+        TaskId = r.GetString(1),
+        Title = r.GetString(2),
+        Completed = r.GetInt64(3) != 0,
+        OrderNum = r.GetInt32(4),
+        CreatedAt = DateTime.Parse(r.GetString(5)),
+    };
+}