policy-engine: add a clear_checks entry point to reset the policy to empty in one call

[Idaonoli]

Scope

Resetting a policy-engine instance's checks today means calling remove_check once per entry, in reverse index order to avoid index-shift bugs. A single clear_checks call would be both more ergonomic and cheaper than N individual removals for an issuer reconfiguring their policy from scratch.

Acceptance criteria

• Add clear_checks(admin) resetting the check list to empty in one call
• Test covers clearing a non-empty policy and confirming get_checks() returns empty afterward