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.

tests/auth/backendContextHeaders.test.ts

@@ -0,0 +1,29 @@
+import { describe, expect, it } from "bun:test";
+
+import { buildBackendContextHeaders } from "../../src/auth/backendContextHeaders.js";
+
+describe("buildBackendContextHeaders", () => {
+  it("forwards authenticated project and trace context to the backend", () => {
+    expect(
+      buildBackendContextHeaders({
+        accessToken: "access-token-1",
+        projectId: "project-id-1",
+        traceId: "trace-id-1",
+      }),
+    ).toEqual({
+      Accept: "application/json",
+      "Content-Type": "application/json",
+      Authorization: "Bearer access-token-1",
+      "X-Project-Id": "project-id-1",
+      "X-Trace-Id": "trace-id-1",
+    });
+  });
+
+  it("omits optional authentication and project headers when unavailable", () => {
+    expect(buildBackendContextHeaders({ traceId: "trace-id-2" })).toEqual({
+      Accept: "application/json",
+      "Content-Type": "application/json",
+      "X-Trace-Id": "trace-id-2",
+    });
+  });
+});