Adds PolicyActionHold so an admin can suspend specific actions on a computer without editing the policy itself, covering the break-glass case where a machine must be quiesced immediately. Holds cover multiple actions in one atomic submission rather than one row per action, so a partial failure cannot leave a half-applied hold. Duration is free-form with 0 meaning indefinite (migration 0006 makes expiresat nullable); the prior fixed ceiling was removed because an admin quiescing a misbehaving machine does not know in advance how long it needs, and a hold silently expiring is worse than one that must be cleared deliberately. The What This Does explainer reflects indefinite holds.
collect_now exemption: refreshing status was denied on any endpoint with a non-empty effective policy even when the caller held the matching grant, because policy intersection silently removed the action. collect_now is a read-only status refresh with no side effects, so it now sits in POLICY_EXEMPT_ACTIONS alongside the other routine actions. Verified against a synthetic non-empty policy: operators can refresh status again while every write action stays gated.
