Add write API to gitlab-policy-store gem

What does this MR do and why?

Extend the gitlab-policy-store gem with write-side operations:

The policy repository contract now defines:

• #create(attributes) - creates and returns a Policy
• #find(id) - retrieves a Policy or raises NotFound
• #delete(id) - removes a Policy or raises NotFound
• #list(organization_id:) - returns policies for an org

Part 2 of the Policy Store v2 MR split (see &22937)

References

Related to #606968

How to set up and validate locally

Example usage in Rails console

# Create a policy
policy = Gitlab::PolicyStore.create(
  organization_id: 1,
  name: "My policy",
  trigger_id: "merge_request"
)

# Find by ID
Gitlab::PolicyStore.find(policy.id)

# List policies for an organization
Gitlab::PolicyStore.list(organization_id: 1)

# Delete a policy
Gitlab::PolicyStore.delete(policy.id)

MR acceptance checklist

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