feat(worker): add TranscriptUsageReader for per-model token usage

Aggregates ~/.claude/projects/**/*.jsonl assistant messages by date/model/
scope (ClaudeDo worktree-or-sandbox cwd vs Other), deduped by requestId
(falls back to message.id), with a per-file length+mtime cache so repeat
calls skip unchanged files.

src/ClaudeDo.Worker/Usage/UsageModels.cs

@@ -0,0 +1,17 @@
+namespace ClaudeDo.Worker.Usage;
+
+public enum UsageScope
+{
+    ClaudeDo,
+    Other,
+}
+
+public sealed record UsageAggregateRow(
+    DateOnly Date,
+    string Model,
+    UsageScope Scope,
+    long InputTokens,
+    long OutputTokens,
+    long CacheReadTokens,
+    long CacheCreationTokens,
+    int Messages);