Migrate `policies` to the new Resource-layer methods

[AlexVelezLl]

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Overview

Migrate the policies plugin to the new resource-layer methods, following the shared recipe in #15057.

Complexity: Low
Target branch: develop

Context

policies currently uses the old resource methods. This task applies the migration recipe from #15057 to this plugin only. The new methods already exist (#15056), and the old ones still work, so this change is isolated to policies.

The Change

Apply the #15057 recipe to every resource usage in policies:

• Replace old base-method calls with the new ones (fetchModel→retrieve, fetchCollection→list, saveModel→create/update, deleteModel→delete, saveCollection→bulkCreate, deleteCollection→bulkDelete, raw get/list/create).
• Remove cache-management calls (clearCache, getModel, getCollection, addModel, …).
• Custom methods: rewrite onto request if the resource is defined and used only within policies; otherwise add/point at a _v2 shared method (see Migrate all plugins and shared packages to the new Resource-layer methods #15057).
• Use useRetrieve / useList for reactive reads, if possible.

Acceptance Criteria

• No code in policies calls the old base methods or cache-management helpers.
• Custom methods handled per the recipe (in-place rewrite for target-only resources, or shared _v2 with the original left intact).
• Reactive reads use useRetrieve / useList where possible.
• Tests that call/mock replaced methods are updated; pnpm test-jest passes for policies.

AI usage

I used Claude Code to help draft this issue from the resource-layer refactor plan. I reviewed and edited it and confirmed the scope myself.