feat(data,worker): add db schema init and signalr hub skeleton

Data layer: Paths helper with ~/%USERPROFILE% expansion, SqliteConnectionFactory
(WAL + foreign keys), SchemaInitializer that applies the embedded schema.sql, and
POCO entities for lists/tasks/tags/worktrees.

Worker: WorkerConfig loader (~/.todo-app/worker.config.json with defaults),
WorkerHub exposing Ping(), and Program.cs wiring Kestrel to 127.0.0.1:<port>,
SignalR at /hub, schema applied on startup.

Pins Microsoft.Data.Sqlite and Microsoft.Extensions.Hosting to 8.x for net8.0
compatibility.

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

src/ClaudeDo.Data/Paths.cs

@@ -0,0 +1,31 @@
+namespace ClaudeDo.Data;
+
+public static class Paths
+{
+    /// <summary>
+    /// Expands a leading "~" or "%USERPROFILE%" and returns an absolute path.
+    /// Relative paths are resolved against <paramref name="baseDir"/> (default: current directory).
+    /// </summary>
+    public static string Expand(string path, string? baseDir = null)
+    {
+        if (string.IsNullOrWhiteSpace(path))
+            throw new ArgumentException("Path must not be empty.", nameof(path));
+
+        var expanded = Environment.ExpandEnvironmentVariables(path);
+
+        if (expanded.StartsWith("~", StringComparison.Ordinal))
+        {
+            var home = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+            expanded = home + expanded[1..];
+        }
+
+        if (!Path.IsPathRooted(expanded))
+            expanded = Path.GetFullPath(expanded, baseDir ?? Environment.CurrentDirectory);
+
+        return Path.GetFullPath(expanded);
+    }
+
+    /// <summary>~/.todo-app — parent directory for db, logs, config, sandbox, worktrees.</summary>
+    public static string AppDataRoot() =>
+        Expand("~/.todo-app");
+}