fix(agent): preserve runtime request context

OpenCode tools run in a child service, so they cannot read the Agent process-local session map. Hydrate a sanitized context through the authenticated internal bridge and centralize backend project headers to prevent context loss across both boundaries.

src/auth/backendContextHeaders.ts

@@ -0,0 +1,25 @@
+import type { RuntimeSessionContext } from "../runtime/sessionContext.js";
+
+type BackendContext = Pick<
+  RuntimeSessionContext,
+  "accessToken" | "projectId" | "traceId"
+>;
+
+export const buildBackendContextHeaders = (
+  context: BackendContext,
+): Record<string, string> => {
+  const headers: Record<string, string> = {
+    Accept: "application/json",
+    "Content-Type": "application/json",
+    "X-Trace-Id": context.traceId,
+  };
+
+  if (context.accessToken) {
+    headers.Authorization = `Bearer ${context.accessToken}`;
+  }
+  if (context.projectId) {
+    headers["X-Project-Id"] = context.projectId;
+  }
+
+  return headers;
+};