Add --batch-size, --output format and a worker --backend flag

[vyncint]

Today — the operational surface is small: oxide has --table, --cluster, --target, --query; environment: OXIDE_BACKEND, OXIDE_CLUSTER_BACKEND, RUST_LOG. The GPU batch size is a constant (crates/oxidelake-storage/src/session.rs:197, GPU_BATCH_SIZE = 65_536); the worker has no --backend flag (env only, bin/oxide_worker.rs:289); there is no --output json|csv and no config file.

Fix — --batch-size (embedded and cluster), --output table|json|csv on oxide sql, --backend on oxide-worker mirroring OXIDE_BACKEND; document every flag and env var in one README table.

Done when — the flags appear in --help with assert_cmd tests; the README table lists every knob.