feat(hub): usage fact ledger for run-scoped cost attribution (ADR-0026) (#4)

Co-authored-by: Hong Jiarong <me@jrhim.com>
Co-committed-by: Hong Jiarong <me@jrhim.com>

hub/src/feishu/trigger.ts

@@ -604,6 +604,32 @@ export function makeTriggerHandler(deps: TriggerDeps): TriggerHandler {
             data: { metadata: mergeSessionMetadata(session.metadata, metadataPatch) },
           });
         }
+        // ADR-0026: the UsageFact ledger is the truth; AgentRun.costUsd /
+        // inputTokens / outputTokens are a derived rollup cache for existing
+        // readers. We write the fact first, then mirror it onto the run.
+        // They are separate statements (not one transaction) so the AgentRun
+        // row lock is held for the shortest possible window and concurrent
+        // workspace teardown cannot deadlock on the FK ShareLock. A crash
+        // between the two leaves the cache stale, but the cache is derived
+        // (the usage service re-reads facts), so staleness is recoverable;
+        // the reverse order would lose the truth and is unrecoverable.
+        const finishedAt = new Date();
+        const reportedCost = result.costUsd;
+        const costSource = reportedCost !== undefined ? "provider_reported" : "unknown";
+        await deps.prisma.usageFact.create({
+          data: {
+            runId: run.id,
+            occurredAt: finishedAt,
+            kind: "model_completion",
+            provider: providerId,
+            model,
+            inputTokens: result.usage.inputTokens,
+            outputTokens: result.usage.outputTokens,
+            costUsd: reportedCost ?? null,
+            costSource,
+            metadata: {},
+          },
+        });
         await deps.prisma.agentRun.update({
           where: { id: run.id },
           data: {
@@ -618,9 +644,10 @@ export function makeTriggerHandler(deps: TriggerDeps): TriggerHandler {
                     : "FAILED",
             inputTokens: result.usage.inputTokens,
             outputTokens: result.usage.outputTokens,
-            ...(result.costUsd !== undefined ? { costUsd: result.costUsd, costSource: "provider_reported" } : {}),
+            costUsd: reportedCost ?? null,
+            costSource,
             error: result.error ?? null,
-            finishedAt: new Date(),
+            finishedAt,
           },
         });
         await writeAudit(deps.prisma, {