Realm _atomic rejects the advertised remove operation #5665

Problem

The public AtomicOperationType includes remove:

export type AtomicOperationType = 'add' | 'update' | 'remove';

but Realm.validate() rejects every operation other than add and update with a 422 validation error. BoxelCLIClient.delete() exists, but it is an immediate standalone request and cannot participate in a coordinated multi-file atomic commit.

This blocks the Realm Program / Realm Script capability from safely providing shell-equivalent fs.remove() and fs.move(). Implementing those with direct deletes would violate preview mode and all-or-nothing commit semantics.

Evidence

• packages/runtime-common/atomic-document.ts: AtomicOperationType includes remove.
• packages/runtime-common/realm.ts: atomic validation only accepts add and update.
• packages/realm-server/tests/atomic-endpoints-test.ts: asserts unsupported operations are rejected.
• packages/boxel-cli/src/lib/boxel-cli-client.ts: delete() is standalone while atomicOperation() forwards to _atomic.

Expected behavior

POST <realm>/_atomic should support a remove operation with the same authorization, realm write lock, validation, indexing, invalidation, notification, and rollback guarantees as add/update.

Once available, Realm Script can implement:

• realm.fs.remove(path) as a staged atomic remove;
• realm.fs.move(from, to) as an atomic add/update + remove;
• preview diffs that represent deletes and moves without mutating the Realm.

Acceptance criteria

• _atomic accepts { op: 'remove', href } without data.
• Missing-resource behavior is defined and tested (prefer a 404 conflict-style failure over silent success unless existing Realm semantics say otherwise).
• Mixed add/update/remove batches are all-or-nothing under the Realm write lock.
• Successful removal triggers the normal index invalidation and Realm notifications.
• BoxelCLIClient.atomicOperation() can submit the operation without a special-case workaround.
• Integration tests cover remove-only, move-shaped add+remove, mixed-operation rollback, authorization failure, and concurrent writes.