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/spawner.ts

@@ -86,6 +86,7 @@ export async function startTask(
 
   const parentSession = await client.session.get({
     path: { id: input.parentSessionID },
+    query: { directory },
   }).catch((err) => {
     log(`[background-agent] Failed to get parent session: ${err}`)
     return null