fix(background-agent): pass query directory to session.get in 4 call-sites (#2937)

resolveSubagentSpawnContext and related lookups called client.session.get
without the query.directory parameter, causing project-scoped sessions
to 404 under newer OpenCode SDK versions. Threaded directory through
SpawnerContext/BackgroundManager so all four call-sites pass it.

🤖 Generated with OhMyOpenCode assistance
https://github.com/code-yeongyu/oh-my-opencode

src/features/background-agent/session-existence.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, mock, test } from "bun:test"
+
+import type { OpencodeClient } from "./opencode-client"
+import { verifySessionExists } from "./session-existence"
+
+describe("verifySessionExists", () => {
+  test("passes query directory to session lookup when provided", async () => {
+    // given
+    const get = mock(async () => ({ data: { id: "session-123" } }))
+    const client = {
+      session: {
+        get,
+      },
+    } as unknown as OpencodeClient
+
+    // when
+    const result = await verifySessionExists(client, "session-123", "/project/root")
+
+    // then
+    expect(result).toBe(true)
+    expect(get).toHaveBeenCalledWith({
+      path: { id: "session-123" },
+      query: { directory: "/project/root" },
+    })
+  })
+})