feat(usage): reproduce TokenTracker session_hash for task attribution

src/ClaudeDo.Worker/Usage/TokenTracker/SessionHash.cs

@@ -0,0 +1,25 @@
+using System.Security.Cryptography;
+using System.Text;
+
+namespace ClaudeDo.Worker.Usage.TokenTracker;
+
+/// <summary>
+/// Reproduces TokenTracker's session identity so its export rows can be joined onto our
+/// <c>task_runs.session_id</c>. The formula is <c>sha256(source + "\0" + id)</c>, hex, first
+/// 24 chars (TokenTracker's <c>lib/session-analytics.js</c>, <c>sessionHash()</c>). The export
+/// never carries the raw session id, so this is the only way to attribute a row to a task.
+/// Verified against a real export on 2026-08-24 — the test vectors are the proof, don't touch them.
+/// </summary>
+public static class SessionHash
+{
+    private const int HashLength = 24;
+
+    public static string? ForClaudeSession(string? sessionId)
+    {
+        if (string.IsNullOrWhiteSpace(sessionId)) return null;
+
+        var bytes = Encoding.UTF8.GetBytes($"claude\0{sessionId}");
+        var hash = SHA256.HashData(bytes);
+        return Convert.ToHexString(hash).ToLowerInvariant()[..HashLength];
+    }
+}

tests/ClaudeDo.Worker.Tests/Usage/TokenTracker/SessionHashTests.cs

@@ -0,0 +1,33 @@
+using ClaudeDo.Worker.Usage.TokenTracker;
+
+namespace ClaudeDo.Worker.Tests.Usage.TokenTracker;
+
+public sealed class SessionHashTests
+{
+    [Theory]
+    [InlineData("77470328-94f7-49d3-b378-a562d1501b5f", "b24babcbf2b615730459773f")]
+    [InlineData("20ebc1bc-3e6a-4c09-a4b3-19472dffa841", "3e2a08fe8f2b941817fb554f")]
+    [InlineData("559e7ebb-7215-4112-9faf-be86bece9613", "03c30f4bc0bfe82c3fbb17c1")]
+    public void ForClaudeSession_MatchesTokenTrackerHash(string sessionId, string expected)
+    {
+        Assert.Equal(expected, SessionHash.ForClaudeSession(sessionId));
+    }
+
+    [Fact]
+    public void ForClaudeSession_IsLowercaseHexOf24Chars()
+    {
+        var hash = SessionHash.ForClaudeSession("any-session-id");
+
+        Assert.Equal(24, hash!.Length);
+        Assert.All(hash, c => Assert.Contains(c, "0123456789abcdef"));
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void ForClaudeSession_BlankId_ReturnsNull(string? sessionId)
+    {
+        Assert.Null(SessionHash.ForClaudeSession(sessionId));
+    }
+}