feat(runtime-local): MONGO_DATABASE env var fallback for mongo store

Resolution precedence inside the bundled harness now reads:
  per-request sessionStore.options.database
    > MONGO_DATABASE env var
    > MongoSessionStore's own default ("computeragent_sessions")

Keeps the database name out of client request bodies in the common
single-database deployment: server holds MONGO_URL + MONGO_DATABASE,
client just sends sessionStore: { kind: "mongo" }.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: kapaleshreyas

packages/runtime-local/assets/harness-bundle.mjs

@@ -180909,9 +180909,10 @@ var app = createHarnessServer({
       if (!url3) {
         throw new Error("mongo session store: MONGO_URL env var or options.url is required");
       }
+      const database = o2.database ?? process.env.MONGO_DATABASE;
       return new MongoSessionStore({
         url: url3,
-        ...o2.database ? { database: o2.database } : {}
+        ...database ? { database } : {}
       });
     }
   },

packages/runtime-local/src/sandbox-boot.ts

@@ -39,9 +39,14 @@ const app = createHarnessServer({
           "mongo session store: MONGO_URL env var or options.url is required",
         );
       }
+      // Database resolution precedence:
+      //   per-request options.database  >  MONGO_DATABASE env  >  store's own default
+      // Keeps the database name out of client request bodies for the common
+      // single-database deployment.
+      const database = o.database ?? process.env.MONGO_DATABASE;
       return new MongoSessionStore({
         url,
-        ...(o.database ? { database: o.database } : {}),
+        ...(database ? { database } : {}),
       });
     },
   },