SetAppStandbyBucketTool

am set-standby-bucket <pkg> <bucket> — set the app's standby bucket, which controls how aggressively the system throttles its background work. Idempotent. The bucket value is lowercased and validated against VALID_BUCKETS (active / working_set / frequent / rare / restricted) before issuing the command; an unknown bucket fails with invalid_args. Treated as a command failure on non-zero exit or non-empty stderr.

Privilege: requires a working ShellBackend.

Params: package_name (required), bucket (required).

On success the result map carries success (true), package_name, and bucket.