BREG task grants: runtime guard on the batch path and two clarity fixes

[jeremi]

Follow-up to #1029.

• execute_batch_after_attempt in crates/registry-breg/src/mutation.rs relies only on the compile-time rule access_profile.task_grant.direct_mutation_forbidden to keep task profiles off the batch path. Refuse grant tokens at its start, as execute_after_attempt does, so a later compiler change cannot open it.
• authenticate in crates/registry-breg/src/auth.rs parses flat direct claims before discarding them for grant tokens, so a malformed flat value on a grant token gives 401 even though the value would be ignored. Skip the flat parse for grant tokens, or document the refusal.
• products/breg/TASK_GRANTS.md: state that registry_request_task_authority rows keep the disclosed subjects until erasure removes them.